Rocks & Minerals A-E

Actinolite has the formula Ca2(Mg,Fe)5Si8O22(OH)2 and a relative hardness of 6.
It commonly occurs in crystalline schists, often being the chief constituent of green-coloured schists and greenstones. It is used somewhat as an asbestos material.

Adamantine refers to a brilliant luster like that of a diamond. Results from a mineral having a high index of refraction.

Adamite has the formula Zn2(AsO4)(OH) and a relative hardness of 4.
It has brilliant fluorescence, is of scientific interest and is much desired by collectors.

Aegirite has the formula NaFe(Si2O6) and a relative hardness of 7.
It is a rock-forming mineral found mainly in rocks rich in soda and poor in silica. Named afterAegir, Icelandic god of the sea.

Agaric Mineral
Agaric Mineral is one of the purest native carbonates of lime. It is chiefly found in the clefts of rocks and at the bottom of some lakes.

Agate is a semi-precious stone comprised mainly of silica. It is a compact variety of chalcedony. It has a pattern of banding which occurs due to chemical admixtures of hematite, limonite and other minerals.

Alabaster is naturally occurring hydrated calcium sulphate.

Albite has the formula NaAlSi3O8 and a relative hardness of 7.
It is widely distributed, rock-forming mineral. Present in pegmatite dikes and may be found in crystals.

Alkaline rock
Alkaline rock refers to any rock which contains more than average amounts of potassium bearing and sodium bearing minerals.

Allanite has the formula (Ce,Ca,Y)2(Al,Fe)3(SiO4)3(OH) and a relative hardness of 6.
It occurs as a minor constituent of many igneous rocks and is frequently associated with epidote. Found in some magnetic bodies.

Allemontite is a natural alloy of arsenic and antimony. It has a relative hardness of 4.

Almandine is a variety of precious garnet, reddish or violet in colour.

Alteration refers to physical or chemical change in a rock or mineral after its original formation. Can result in new minerals or in textural changes in the rock.

Alumina is aluminium oxide and occurs as ruby, sapphire, and bauxite.

Aluminium is a light-weight silvery metal element with the symbol Al.

Alunite has the formula KAl3(SO4)2(OH)6 and a relative hardness of 4.
It is usually formed by sulphuric acid solutions acting on rocks rich in alkalic feldspar. Used in the production of alum.

Also called Amazonstone, amazonite is a green microcline with the formula KaISI3O8.
Amber is fossilised resin.

Amblygonite has the formula (Li,Na)Al(PO4)(F,OH).
It has a relative hardness of 6.
It occurs in lithium and phosphate rich granite pegmatites. Associated with spodumene, apatite, tourmaline, lepidolite.

Amethyst is a precious stone. It is quartz coloured with manganese.

Amorphous refers to 'Without form'. The term is applied to rocks and minerals that lack definite crystal structure.

The name of a mineral group or "family." All the members have as their basic structure the formula Si4O11. Hornblende is one of the most common rock forming minerals of the group.

Amygdule refers to a mineral containing cavity in an igneous rock formed by escaping gas.

Analcime is a colorless tectosilicate mieneral that consists of hydrated sodium aluminum silicate in a cubic crystalline form, and a relative hardness of 6.
It is found in the cavities of intrusive and volcanic igneous rocks; often as clear shiny crystals and is associated with calcite and zeolites.

Anatase has the formula TiO2 and a relative hardness of 6.
It is a form of rutile found in granite, gneiss, mica schist, maetamorphic limestone, and dolomite. May be present as an accessory mineral in the rocks or in a quartz vein traversing it. It is used as a coating for welding rods and as a derivative of titanium.

Andalusite has the formula Al2SiO5.
It has a relative hardness of 8.
It is formed by the metamorphism of aluminous shales and slate. Used in the manufacture of spark plugs and other porcelains. May serve as a gem stone if clear and transparent. Name from Andalusia, a province of Spain.

Andesine has the formula NaAlSi3O8.
It has a relative hardness of 6.

Anglesite is a common, minor ore of lead formed by the oxidation of galena.It has the formula PbSO4 and a relative hardness of 3.
It is found in the upper, oxidzed portions of lead veins and is named after the Island of Anglesey.

Anhydrite has the formula CaSO4.
It has a relative hardness of 4.
It has rectangular cleavage fragments. Occurs in a manner similar to gypsum and often found with it but not as common. Found in the cap rock of salt domes and in limestone rocks. From the Greek meaning 'without water'.

Annivite is a variety of terahedrite containing bismuth and usually iron and zinc.

Anorthite has the formula (Na,Ca)AlSi3O8.
It has a relative hardness of 7.
It is widely distributed and abundant rock-forming feldspar.

Anthophyllite has the formula (Mg,Fe)7Si8O22(OH)2.
It has a relative hardness of 6.
It occurs in crystalline schists rich in magnesium. Of interest to collectors.

Antimonite is an antimony ore. It was used by the Greeks as a cosmetic for darkening the eyelids.

Antimony has the formula Sb.
It has a relative hardness of 4.
It is a native semimetal and difficult to distinguish from arsenic. Rather brittle and a poorer conductor of heat and electriciy than native metals.

Apatite has the formula Ca5(F,Cl,OH)(PO4)3.
It has a relative hardness of 5.
It is widely distributed as an accesory constituent in all classes of rock. Found in crystals and masses. Crystallized apatite has been extensivley for fertilizer. Transparent varieties of fine colour are sometimes used for gems - but it's too soft for extensive use.

Aphanitic rock
Aphanitic rock refers to a rock in which the crystalline constituents are too small to be distinguished without magnification.

Apophyllite is a phyllosilicate.
It has a relative hardness of 5.
It is a hydrothermal mineral filling cavities in basalt and tufaceous rocks. Associated with stilbite, scolecite, calcite, prehnite, analcime.

Aragonite has the formula CaCO3.
It has a relative hardness of 4.
It forms easily in a sedimentary environment but only stable in metamorphic rocks formed at high temperatures.

Arfvedsonite has the formula Na2-3(Fe,Mg,Al)5Si8O22(OH,F)2.
It has a relative hardness of 6.
It is typical of alkaline plutonic rocks that are rich in iron.

Argentite has the formula Ag2S.
It has a relative hardness of 3.
It lacks distinct cleavage. An important primary silver ore. The name is from Latin meaning 'silver'. Occurs disseminated in galena and in the cementation zone of lead and zinc deposits. Associated with cerussite, chlorargyrite, native silver.

Argillaceous refers to composed mostly of or containg clay; such as shale.

Arsenates refers to minerals in which arsenate (AsO4) is an important part.

Arsenic is a metal element with the symbol As and a relative hardness of 4.
It has a garlicky odor; otherwise it is difficult to distinguish from antimony. It is a relatively rare mineral found in veins in crystalline rocks associated with silver, cobalt, or nickel ores.

Arsenopyrite has the formula FeAsS.
It has a relative hardness of 6.
It is the most common mineral containing arsenic. It occurs with tin and tungsten ores in high temperature deposits as a deposition from hot waters. Frequently associated with gold. Also found with copper and silver ores.

Asbestos is a fibrous mineral with a high melting and combustion point. Cloth was made of it by the ancient Egyptians.

Atacamite has the formula Cu2Cl(OH)3.
It has a relative hardness of 4.
It is a comparatively rare copper mineral. Occurs in arid regions in the upper oxidized zone of copper deposits.

Augite has the formula (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6.
It has a relative hardness of 6.
It is an important rock forming mineral. Chiefly found in dark coloured igneous rocks, especially those whose magmas were rich in iron, calcium and magnesium. Seldom found in rocks that contain much quartz. A common memeber of the pyroxene group. Told from amphibole by cleavage.

Autunite is a radioactive orthorhombic mineral created from the hydrothermal alteration of uranioum minerals. It is a hydrated calcium uranyl phosphate.
It has a relative hardness of 3.
It has strong fluorescence. A secondary mineral found in the zone of oxidation and weathering of uranite or other uranium minerals. Used as an ore of uranium.

Axinite has the formula Ca2(Fe,Mn)Al2(BO3)(Si4O12)(OH).
It has a relative hardness of 7.
It occurs in cavities in granite and in the contact zones surrounding granite intrusions.

Azurite has the formula Cu3(CO3)2(OH)2.
It has a relative hardness of 4.
It is a minor ore of copper with an intense azure-blue colour. Alters to malachite. Associated with limonite, calcite, chalcocite, chrysocolla and other secondary copper minerals. Reacts vigorously with hydrochloric acid.

Barite has the formula BaSO4.
It has a relative hardness of 3.
It is major ore of barium. Has a high specific gravity for a light coloured mineral. Common gangue mineral in hydrothermal veins or as a repacement mineral in veins of limestone and dolomite. Associated with lead, silver and antimony sulphides.

Barium is a metal element of the alkaline earth group with the symbol Ba.

Barytes was the first mineral to be found to be luminescent when heated, and led to the discovery of the luminescence of minerals. It has a relative hardness of 3.5.

Basalt is the name given to lava. It is high in ferrous and magnesian silicates.

Basanite is a smooth, black siliceous mineral.

Basic rock
Basic rock refers to an igneous rock with a low percentage of silica and a high percentage of pyroxene, hornblende, and labradorite.

Bauxite is a sedimentary rock group of various aluminium oxides, a principal ore of aluminium, found in France and Jamaica. Bauxite was named after the place where it was first found, Les Baux in France. It has a relative hardness of 1 to 3.

Bedding refers to the arrangement of sedimentary rocks in about parallel layers or strata which correspond to the original sediments.

Beryl (beryllium aluminium meta-silicate) has the formula Be3Al2Si16O18 and a relative hardness of 8.
It has a characteristic six-sided outline and is used as a gem stone of various colours, its green variety being emerald. Beryl is also the major source of the rare element beryllium, a light metal similar to aluminum. Beryl is quite common and occurs usually in granite rocks, mica schists and with tin ores.

A rare, lightweight metal element found in the earth's crust with the symbol Be.

Beryllium aluminium meta-silicate
see "Beryl"

Biotite has the formula K(Mg,Fe)2(Al,Fe)Si3O10(OH,F)2 and a relative hardness of 3.
It is a widely distributed rock forming mineral and occurs in igneous and metamorphic rocks and is a common member of the mica group.

Bismuth is a metal element with the symbol Bi and a relative hardness of 3. Bismuth is a somewhat rare mineral occurring in connection with ores of silver, cobalt, nickel, lead, and tin. Bismuth forms low melting alloys and is used in fuses, medicine and cosmetics.

Bismuthinite has the formula Bi2S3 and a relative hardness of 2. It is a rare ore of bismuth. Occurs in veins that show definite relations to igneous rocks.

Bituminous rocks
Bituminous rocks refers to rocks that contain tar, petroleum, or asphalt.

Blende is a zinc ore. It generally contains more than half its weight in zinc, a quarter sulphur and often a small amount of iron.

Bloodstone is a dark green variety of chalcedony.

Bohemian garnet
see "Pyrope"
Bole is an earthy mineral occuring in amorphous masses , and composed chiefly of silica with alumina, iron and occasionally magnesia.

Boracite has the formula Mg3B7O13Cl and a relative hardness of 7. It occurs associated with beds of halite, anhydrite, and gypsum. It is formed by the evaporation of bodies of salt water.

Borates refers to a group of minerals in which the borate radical (BO3) is an important constituent.

Borax (sodium tetraborate) is the sodium salt of pyroboric acid. It has the formula Na2B4O710H2O and a relative hardness of 3. It forms large transparent six-sided prisms which have an alkaline reaction, effloresce in air, and when heated swell-up and melt to a transparent glass. Borax is used in the manufacture of enamel-ware, glass, as an antiseptic and is a food preservative.

Bornite has the formula Cu5FeS4 and a relative hardness of 3. It is an ore of copper with a colourful tarnish, widely occurring, it is found in basic rocks and metamorphic deposits.

Boron is an element with the symbol B found in borax.

Botryoidal refers to resembling a bunch of grapes. A mineral of this type appears to have a surface covered with spherical bulges.

Boulangerite is a lead ore. It contains 55 percent lead. It has a relative hardness of 2.5.

Bournonite has the formula PbCuSbS3.
It has a relative hardness of 3.
It is an ore of lead, copper, and antimony which often exhibits twinned crystals. Occurs in in veins formed at moderate temperatures.

An alloy of copper and tin discovered during the Bronze Age (6,000-1,000 BC). Harder and more durable than copper by itself and more easily workable than most other known metals and alloys of the period. It was used in making tools, weapons and decorative articles.

Brimstone is another name for sulphur.

Bronze is an alloy of copper, zinc and tin.

Brookite is a source of titanium but deposits are usually too small to be of commercial use. It has the formula TiO2 and a relative hardness of 6.

Brown Spar
Brown Spar is the name given to some crystalline varieties of dolomite tinged with peroxide of Iron

Brucite is a decomposition product of magnesium silicates, especially serpentine. It has the formula Mg(OH)2 and a relative hardness of 3.
It is found in Texas, where it is white with a grey, green or blue tinge and is used in sugar-refining.

Cadmium is a metal element with the symbol Cd.

Calamine is a zinc ore.

Calcareous refers to containing calcium carbonate or calcite.

Calcic refers to containing calcium.

Calcite has the formula CaCO3.
It has a relative hardness of 3.
It effervesces vigorously with hydrochloric acid (HCl). Clear specimens exhibit double refraction. Occurs as widespread sedimentary rock masses such as limestone. Crystalline metamorphosed limestones are called marbles. Fine grained deposits form chalk. Variety: onyx.

Calcium is a naturally occurring element with the symbol Ca.

Caliche is naturally occurring sodium nitrate found in Chile.

Cannel Coal
Cannel Coal is a dull black coal which breaks with a conchoidal fracture and does not soil the fingers when handled. It is easily cut, and will take a high polish. It contains a large proportion of volatile constituents making it suitable for gas manufacture, and it burns with a bright white flame.

Carbon is a non-metallic element with the symbol C. Contained in all life forms.

Carbonaceous refers to composed chiefly of organic carbon. (i.e. carbon derived from plant and animal remains.)

Carbonates refers to minerals, such as calcite, where the carbonate radical (CO3) is an important constituent.

Carnallite is a source of potassium coumpounds and magnesium. It is an evaportite mineral that has a bitter, salty taste and has a relative hardness of 3.

Carnelian is a clear red chalcedony, a semi-precious gemstone, consisting of quartz with iron impurities which give it a translucent red colour. Carnelian is found mainly in Brazil, Japan and India.

Carnotite has the formula K2(UO2)2 (VO4) 3H2O.
It has a relative hardness of 1.
It is an ore of uranium and vandium. Found in sand, sandstones and around petrified trees. Probably formed as a deposition from meteoric waters. Strongly radioactive.

Cassiterite has the formula SnO2 and a relative hardness of 7.
It is the principal ore of tin. Widely distributed in small amounts but commerically available in only a few localities. Frequently associated with wolframite. Also found as rolled pebbles in placer deposits ('stream tin'). Usually found in veins associated with quartz, in or near granitic rocks.

Celestine is a natural sulphate of strontium.

Celestite has the formula SrSO4.
It has a relative hardness of 4.
It often found disseminated through limestone or sandstone, or lining cavities in such rocks. Associated with calcite, dolomite, gypsum, sulphur, fluorite. Also found as a gangue mineral in lead veins. Used to prepare nitrate of strontium for fireworks and tracer bullets and in the refining of beet sugar.

Cerium is a rare metal element with the symbol Ce.

Cerussite has the formula PbCO3.
It has a relative hardness of 4.
It is an important and widely distributed supergene lead ore formed by the action of carbonated waters on galena in the upper zone of lead veins. Often found associated with galena and sphalerite.

Chabazite is a tectosilicate mineral of the zeolite group..
It has a relative hardness of 5.
It is a mineral of secondary origin found lining cavities in volcanic and intrusive igneous rocks.

Chalcanthite has the formula CuSO4 5H2O.
It has a relative hardness of 3.
It dissolves quickly in water and produces a blue solution. A minor ore of copper and found only in arid regions. Occurs near the surface in copper veins. Often deposited on iron from the water in copper mines. Used in calico printing, insecticides and for industrial purposes.

Chalcedony is a variant of quartz comprised of silica. Chalcedony was named after Chalkedon, near Istanbul, Turkey. It was traditionally used for decorative objects and amultes. It has a relative hardness of 7.

Chalk is a pure soft limestone, opaque white, and usually formed by the accumulation of the shells of foraminifera together with those of larger marine organisms.

Chalcocite has the formula Cu2S.
It has a relative hardness of 3.
It is one of the most important copper ore minerals. It occurs primarily in enriched zones of sulphide deposits.

Chalcopyrite has the formula CuFeS2.
It has a relative hardness of 4.
It is the most widely occuring copper mineral and one of the most important ore sources of that metal. Occurs as an original constituent of igneous rocks, in pegmatic dikes, and in contact with metamorphic deposits. May carry gold or silver and become an ore of those metals.

Chalcosine is an important copper ore. It was discovered in the 16th century.

A mineral in which silica was precipitated in sea water or in which silica replaced existing limestone resulting in a very hard (hardness 7) and durable material. Flint is a form of chert. Used from the stone age until the mid-1800's for making tools, weapons and for starting fire when struck with steel.

Chili Saltpetre
see "Cubic Nitr"
Chlorite has the formula (Mg,Fe)6(AlSi3)O10(OH)8.
It has a relative hardness of 3.
It is a mineral group whose members usually exhibit a charactersitic green colour. The formula above is for 'green mica'. Distinguished from muscovite and green phlogopite by a lack of elasticity.

Chondrodite has the formula (Mg,Fe)3(SiO4)(OH,F)2.
It has a relative hardness of 7.
It is a common metamorphic mineral in dolomitic marbles.

Chromite has the formula FeCr2O4.
It has a relative hardness of 6.
It is slightly magnetic. The only ore of chromium. A common constituent of peridotite rocks and the serpentines derived from them. Also associated with corundum. One of the first minerals to separate from a cooling magma. Chromium is widely used in metal plating and in stainless steel.

Chromium is a metal element with the symbol Cr.

Chrysoberyl has the formula BeAl2O4.
It has a relative hardness of 9.
It occurs in granite rocks, pegmatites, and in mica schists. Frequently in river sand and gravels. Serves as a gem stone: alexandrite and "cats eye" which can be of great value.

Chrysocolla has the formula (Cu,Al)2H2Si2O5(OH)4 nH2O.
It has a relative hardness of 4.
It is a minor ore of copper. A mineral of secondary origin, occuring in the oxidized zones of copper veins. Associated with malachite, azurite, cuprite, native copper. Named from two Greek words meaning 'gold' and 'glue'.

Chrysostom is a gem stone of beryllium aluminate.

Cinnabar is a red crystalline form of mercuric sulphide. It has the formula HgS and a relative hardness of 3.
It often has a bright red colour. The only important source of mercury and it's found in few localities. Occurs as vein fillings near recent volcanic rocks and hot springs. Used in scientific equipment, in drugs, and with tin in silvering mirrors. Many other uses.

Clastic rock
Clastic rock refers to a sedimentary rock comprised of fragments of preexisting rocks that have been transported and deposited.

Clay refers to a soft sediment or deposit that is plastic when wet and comprised of very fine-grained materials, mainly hydrous aluminum silicates.

Cleavage refers to a mineral is said to possess cleavage if when it breaks it yields definite plane surfaces. Cleavage can be perfect as in micas or, in some minerals, completely lacking. Cleavage is always parallel to crystal faces.

Cleveite is a uranium-containing mineral, of interest owing to the fact that when heated with dilute sulphuric acid it liberates considerable quantities of occluded helium. It was this property which first led to the discovery of the element helium.

Clinozoisite has the formula Ca2Al3Si3O12(OH).
It has a relative hardness of 7.

Coal is a solid combustible material of vegetable origin occurring in a fossilised state.

Coal-tar is a thick black viscous liquid produced by the destructive distillation of coal. Coal-tar yields benzene, creosote, paraffin, aniline and dyes.

Cobalt is a metal element with the symbol Co. It was discovered among the ore veins in Cornwall in early times and called mundic by the miners. It was identified as a metal in 1733 by Brandt.

Cobalt glance
see "cobaltite"
Cobaltite (cobalt glance) is an ore of cobalt consisting of a cobalt arsenosulphide of the formula CoAsS and has a relative hardness of 6.
It is usually found in high temperature deposits, disseminated in metamorphic rocks, or in vein deposits with other cobalt and nickel minerals.

Coelestine is a mineral used in pyrotechnics and a s a source of strontium. Occassionaly it is cut and used as a gem stone. It has a relative hardness of 3.5.

Colemanite is a major source of borax and has a relative hardness of 5. It occurs in high temperature hydrothermal veins or disseminated in metamorphic rocks associated with other cobalt and nickel sulphides/arsenides.

Columbite has the formula (Fe,Mn)(Nb,Ta)2O6 and a relative hardness of 6.
It shows a bluish iridescent fracture surface. The main ore of niobium and tantalum; used in metallurgy to create heat-resistant alloys and in the rust proofing of stainless steel.

Concretion refers to an accumulation of mineral matter when mineral particles become cemented together into an orderly, rounded form.

Contact metamorphism
Contact metamorphism refers to metamorphism resulting from the intrusion of magma which takes place at or near the contact point with the molten rock.

Copper has the formula Cu and a relative hardness of 3.
It is one of the essential metals of modern civilization. Native copper is found in copper veins but copper sulphides are the principal source ores of the metal.

Cordierite has the formula Mg2Al4Si5O18.
It has a relative hardness of 8.
It is found as an accessory mineral in granite, gneiss, schists, and in contact metamorphic zones. Transparent specimens of good colour have been used as a gem.

Corundum has the formula Al2O3.
It has a relative hardness of 9.
It is common as an accessory mineral in metamorphic rocks and as an original constituent of certain igneous rocks. Color differences give rise to several varieties of gem, notably, ruby and saphire. The deep red ruby is one of the most valuable gems, second only to emerald and diamond.

Covellite has the formula CuS.
It has a relative hardness of 2.
It has an indigo-blue colour. Not an abundant material but is found in most copper deposits, usually as a coating in the zone of sulphide enrichment.

Cristobalite has the formula SiO2.
It has a relative hardness of 7.
It is present in many siliceous volcanic rocks as a lining in cavities. Upon heating to 1470 C it becomes nearly transparent. On cooling it assumes its initial white translucent appearance.

Crocoite has the formula PbCrO4.
It has a relative hardness of 3.
It is a rare mineral found in the oxidized zones of lead deposits where lead veins have traversed rocks containing chromite. Not abundant enough to be of commercial value although it does contain chromium. Name is Greek and means ' saffron', an allusion to the colour.

see "Harmotome"

Cryolite has the formula Na3AlF.
It has a relative hardness of 3.
It often found enclosing brown siderite and grey galena. Always occurs in pegmatites where it's probably a precipitate from flouride rich solutions. Used in the manufacture of sodium salts, certain kinds of glass and porcelain, and as a flux for cleaning metal surfaces.

Crystal refers to a solid mineral having a regular geometric shape and bounded by smooth flat surfaces(called crystal faces).

Crystal symmetry
Crystal symmetry refers to the repetitive pattern of crystal faces caused by the orderly internal arrangements of atoms within a mineral.

Cubic Nitre
Cubic Nitre (sodium nitrate,Chili Saltpetre) is a mineral found mainly in the Tarapaca district of Chile.

Cuprite has the formula Cu2O.
It has a relative hardness of 4.
It is an important ore of copper. Found in the upper oxidized portions of copper veins. Commonly found in crystal forms.

Danburite has the formula CaB2Si2O8.
It has a relative hardness of 7.

Datolite has the formula CaBSiO4(OH).
It has a relative hardness of 6.
It is a mineral of secondary origin found usually in cavities in basalt lavas and similar rocks.

Delvauxite is a mineral source of phosphate. It has a relative hardness of 2.5.

Destinezite is a mineral. It is a source of phosphate. It has a relative hardness of 3.

Detrital sediment
Detrital sediment refers to deposited rock and mineral fragments.

Diamond has the formula C.
It has a relative hardness of 10.
It is the hardest naturally occuring mineral and the most important of the gemstones. Occurs in pale shades of several colours, deep shades are rare. Gem quality diamond is transparent, of attractive colour, and without internal fractures or inclusions of other materials.

Diaspore has the formula AlO(OH).
It has a relative hardness of 7.
It is a constituent of bauxites and a major source of aluminum.

Dike refers to a wall-like body of igneous rock that cuts across layers of surrounding rocks.

Diopside has the formula CaMgSi2O6 and a relative hardness of 6.
It is usually found as a contact metamorphic mineral in crystalline limestones. Transparent varieties have been cut and used as gemstones.

Dioptase has the formula CuSiO2(OH)2 and a relative hardness of 5.
It is found in the oxidation zone of copper deposits, in cavities of massive copper minerals.

Diorites are a group of rocks composed essentially of a soda-lime felspar and hornblende, embracing a wide range of types from acid to basic. Diorites were formed by cooling far below the surface and occur in the Scottish Highlands, the Channel Islands, California and other places.

Dolomite has the formula CaMg(CO3)2 and a relative hardness of 4.
It's powder reacts vigorously with hydrochloric acid (HCl). A potential ore of magnesium. Occurs in widely extended rock masses as dolomitic limestone. Often intimately mixed with calcite. Formed from ordinary limestone by the replacement of calcium by magnesium.

Dumortierite has the formula Al7(BO3)(SiO4)3O3 and a relative hardness of 7.
It often has a bright colour and fibrous habit. Occurs in metamorphic rocks rich in aluminum. Also in pegmatites and contact metamorphic rocks.

Dysodile is a yellow or green foliated mineral found in limestone.

Emerald is a green precious stone variety of the mineral beryl.

Emery is an impure fine-grained aluminium oxide with the formula Al2O3 employed widely as an abrasive. Typically emery consists of 60 percent corundum and 40 percent iron oxide in the form of magnetite.

Enargite has the formula Cu3AsS4.
It has a relative hardness of 4.
vein and replacement deposits associated with pyrite, shpalerite, bornite, galena, chalcocite. Used as an ore of copper.

Enstatite has the formula Mg2Si2O6.
It has a relative hardness of 6.
It is a common and widespread mineral. Found in mafic plutonic and vocanic rocks and often in both metallic and stony meteorites.

Epidote has the formula Ca2(Al,Fe)3Si3O12(OH).
It has a relative hardness of 7.
It is a common mineral. Found in regional and metamorphic rocks of mafic composition and as a product of alteration of other minerals. Sometimes used as a gemstone.

Epsomite has the formula MgSO4 7H2O.
It has a relative hardness of 3.
It has a bitter, salty taste; dissolves easily in water. Occurs in delicate fibrous and capillary aggregates. Often called 'epsom salt'. Found as an efflorescent deposit on the walls of caves and sometimes in lake deposits.

Erbium is a metal element with the symbol Er.

Erythrite has the formula Co3(AsO4)2 8H2O.
It has a relative hardness of 3.
It often shows a vivid colour and occurs in thin crusts. An important indicator of cobalt mineral deposits.

Euclase has the formula BeAlSiO4(OH).
It has a relative hardness of 8.
It is a variety of berly.