The Mineral Identification Key

Table IIIB-2: Nonmetallic Luster; Hardness 7 to 10; Cleavage not prominent.. (Can not be scratched by quartz.) [Previous Table]

Hardness	Color	Luster	Name	System	Habit	SG	Notes
6 to 7	Black to Dark-brown, may also be Yellowish-grey, more rarely Red, White or Colorless	Adamantine to metallic in crystals, greasy on fracture surfaces, may be earthy or submetallic in botryoids, concretions, and massive forms	CASSITERITE (Rutile Group) SnO2	Tetragonal	Usually massive as botryoidal crusts or concretions ("wood tin"), crystals usually short prismatic and complexly twinned producing unusual shapes ("knees", stubby five-pointed "stars," etc.)	6.8 to 7.1
6½ to 7	Grey to Bluish-grey, Brown to Honey-brown or Yellow to Golden-brown, more rarely Green or Violet	Vitreous to sub-vitreous	FERRO-AXINITE/ MANGANAXINITE Ca2FeAl2BSi4O15(OH) Ca2MnAl2BSi4O15(OH)	Triclinic	Usually as thin wedge-shaped "axhead" crystals, often arranged in rosettes	3.23 to 3.32 (ferro-), 3.30 to 3.36 (mangan-)	End members difficult to distinguish, though low end and high end S.G. may do the trick.  Magnesio-axinite and tinzenite are two rare related species.
6½ to 7	Pale-yellow to Olive-green to Olive-brown, Black	Vitreous to sub-vitreous (forsterite) or submetallic to dull (fayalite)	FAYALITE/ FORSTERITE (Olivine Group) Fe2(SiO4)  Mg2(SiO4)	Orthorhombic	Usually as crystalline massive or granular, crystals short-prismatic	4.39 (fayalite) to 3.24 (forsterite)	End members distinguished by S.G. and luster.  "Peridot" is the name for gem material in the fayalite-forsterite series, usually forsterite.  Names for intermediate members of the series, such as "chrysolite" and "hortonolite" are based on chemical composition and should not be used unless the composition is known.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
6½ to 7	Colorless to White, more rarely Pale-yellow, Pale-pink or Pale-brown	Vitreous	BERTRANDITE Be4Si2O7(OH)2	Orthorhombic	Usually as tiny to micro tabular crystals, also granular aggregates	2.57 to 2.63	Usually associated with beryl in pegmatites.  May exhibit basal and prismatic cleavages.
7	Colorless, Milky, Smoky-grey to Black, Amethyst, Rose, Yellow to Brownish-yellow, may be tinted other shades by inclusions	Vitreous	QUARTZ (Crystalline) SiO2	Trigonal	Usually in pyramidally terminated prismatic hexagonal crystals or glassy massive, "rock crystal," "smoky" and "amethyst" varieties may be very large, "citrine" usually small to medium in size, "rose" usually rather small to micro-scale	2.65	Abundant! in milky and massive forms. Common in rock crystal, smoky and amethyst forms, citrine rare, rose crystals very rare. (See also immediately below for crypto- and non-crystalline forms.)
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7	Colorless, Milky, Grey to Black, Red, Yellow to Brownish-yellow, Brown, Green, Blue, may be tinted other shades by inclusions	Vitreous to waxy to dull	QUARTZ (Crypto- and Non-crystalline) SiO2	Trigonal	"Chalcedony" as cryptocrystalline fibrous amorphous to sub-botryoidal masses, un-banded or un-patterned, with a waxy luster ("carnelian" is red, "chrysoprase" is green due to nickel, "heliotrope" or "bloodstone" is green with small red jasper flecks in it, "sard" is brown); "chalcedony, sub-variety agate" in circular, concentrically banded to intricately patterned masses with considerable color variation in single samples, waxy to vitreous luster; "chalcedony sub-variety onyx" has parallel layers instead of concentric ones ("sardonyx" has alternating white and black layers).	2.65	In massive, non- crystalline forms: "Jasper" red or blue, rarely yellow, dull luster; "flint" dark grey to black, dull to sub-vitreous luster; "chert" medium to light grey, dull to sub-vitreous luster; "aventurine" green due to inclusions, vitreous luster.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7	Colorless, Milky-white, Pale-grey, Yellowish, and Brownish	Vitreous	CRISTOBALITE SiO2	Tetragonal (pseudo Isometric)	Usually as micro-spherical aggregates, may be botryoidal or stalactic, crystals very rare, usually micro-octahedra	2.2 to 2.33	Largely restricted to siliceous volcanic rocks. Cristobalite is only partially stable at typical terrestrial temperatures, inverting to quartz.  Most finds are actually quartz pseudomorphs of the original crystals
7	Colorless, Milky-white	Vitreous	TRIDYMITE SiO2	Orthorhombic & Hexagonal	Usually as thin to thick tabular or twinned pseudo-hexagonal crystals	2.26 to 2.33	Largely restricted to felsic volcanic rocks; Tridymite is only partially stable at typical terrestrial temperatures, inverting to quartz.  Most finds are actually quartz pseudomorphs of the original crystals
7 to 7½	Black to Bluish-black or Brownish-black or Greenish-black	Vitreous to sub-vitreous, may appear somewhat oily on fracture surfaces	SCHORL / UVITE (Tourmaline Group) NaFe3Al6 (BO3)3(Si6O18)(OH)4 (Ca,Na)(Mg,Fe)3Al5Mg(BO3)3(Si6O18)(OH,F)4	Trigonal	Schorl usually in stout to long-prismatic six-sided crystals with a nearly triangular cross-section. Uvite usually in stubby short-prismatic crystals, sometimes appearing almost pyramidal or dipyramidal, but may take identical habits. Both may also be massively crystalline or granular.	3.13 (schorl) 2.96 to 3.06 (uvite)	These two end members are best distinguished by their S.G.s.  Foitite, feruvite, and povondrite are three rare to very rare black Tourmaline Group members.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7 to 7½	Black to Brownish-black or Brown	Vitreous to sub-vitreous, may appear oily on fracture surfaces	DRAVITE (Tourmaline Group) NaMg3Al6(BO3)3(Si6O18)(OH)4	Trigonal	Usually as stout prismatic six-sided crystals, often with a pyramidal termination at one end and a pedial termination at the other	2.9 to 3.3	Largely restricted to marbles, schists and slates.  Best distinguished from schorl and uvite by environment when black. Buergerite is a rare bronze-brown Tourmaline Group member.
7 to 7½	Dark emerald-green to Greenish-black to Black	Vitreous, may appear oily on fracture surfaces	CHROMEDRAVITE (Tourmaline Group) NaMg3(Cr,Fe)6 (BO3)3 (Si6O18)(OH)4	Trigonal	Usually as small to tiny nearly equant short-prismatic crystals and grains	3.39 to 3.40	Usually distinguished by its deep emerald green color – largely restricted to chromium bearing micaceous metasomatites.  Very rare.  Distinguished from dark greenish-black schorl or uvite by its higher S.G
7 to 7½	Light- to Medium-green, Pink to Red, Light- to Medium-blue to Dark-blue, more rarely Light-brown to Yellow, Colorless	Vitreous to oily	ELBAITE (Tourmaline Group) Na(Li,Al)3Al6 (BO3)3(Si6O18)(OH)4	Trigonal	Usually in slender prismatic six-sided crystals with a nearly triangular cross-section, often appearing somewhat rounded. Whole crystals may be hemimorphic, with different shaped terminations (pyramidal and pedial) at opposite ends	3.05 to 3.10	Distinguished from most other Tourmaline Group species by its lighter colors – also, largely restricted to granite-pegmatites.  Dark-blue elbaite is almost impossible to distinguish from dark blue schorl, and both can occur together.  Olenite, pale pink, and liddicoatite, light-pink or light-green, are two rare to very rare colored Tourmaline Group species.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7 to7½	Deep-green to "Rusty"-green	Vitreous	UVAROVITE (Garnet Group) Ca3Cr2(SiO4)3	Isometric	Usually as small to tiny dodecahedral crystals	3.40 to 3.83	Largely restricted to chrome-bearing deposits, particularly serpentinite ultramafics containing chromite.  Rare. Goldmanite and knorringite are very rare dark-green and blue-green Garnet Group species, but are found in different mineral environments than uvarovite.
7 to7½	Pink to Red, Brownish-orange to Orange, Cinnamon, Yellow to Yellow-green, Pale-green to Grass-green to Emerald-green, Colorless	Vitreous	GROSSULAR (Garnet Group) Ca3Al2(SiO4)3	Isometric	Usually as small dodecahedral crystals	3.42 to 3.80	Largely found in contact and regionally metamorphosed impure calcareous rocks, and in serpentinite and rodingite ultramafic rocks.  Hibschite and katoite are very rare Garnet Group species that can be similar greens or milky to colorless, but they are found in different mineral environments than grossular.
7 to7½	Brown, Brownish-red to Brownish-yellow, Yellow to Yellow-green, Greyish-green to Medium- or Deep-green, Black	Vitreous	ANDRADITE (Garnet Group) Ca3Fe2(SiO4)3	Isometric	Usually as small to medium sized dodecahedral crystals	3.45	Usually from Ca and Fe rich contact metamorphic rocks and skarns, also from alkaline and ultrabasic igneous rocks.  Calderite is a very rare Garnet Group species with color similar to andradite, but it is found in a different mineral environment.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7 to 7½	Black to Brownish-black	Vitreous	SCHORLOMITE (Garnet Group) Ca3Ti2(Fe2Si)O12	Isometric	Usually as small dodecahedral crystals	3.77 to 3.93	Largely restricted to alkaline igneous rocks.  Morimotoite and kimzeyite are very rare Garnet Group species that may be the same color as schorlomite, but they are found in different environments
7 to 7½	Dark-red to Violet-red or Deep rose-red, also Reddish-orange	Vitreous	PYROPE (Garnet Group) Mg3Al2(SiO4)3	Isometric	Usually as small to tiny dodecahedral crystals	3.65 to 3.82	Usually found in high temperature and high pressure metamorphic rocks, ultrabasic igneous rocks, and kimberlites and peridotites.  Uncommon.  Majorite is a very rare Garnet Group species that can be purplish-violet, but is found in a different environment than pyrope.
7 to 7½	Brown to Brownish-red to Red, Violet-red or Orangish-red	Vitreous	ALMANDINE (Garnet Group) Fe3Al2(SiO4)3	Isometric	Usually as dodecahedral crystals	3.85 to 4.20	Occurs in a number of metamorphic and igneous environments – most common of the Garnet Group species.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7 to 7½	Orangish-red to Reddish-orange, Rose-red to Ruby-red or Hyacinth-red, Reddish-brown, Pale-yellow	Vitreous	SPESSARTINE (Garnet Group) Mn3Al2(SiO4)3	Isometric	Usually as medium to tiny dodecahedral crystals	3.90 to 4.20	Occurs in metamorphic and igneous rocks rich in Mn.
7 to 7½	Medium- to Dark-brown, Reddish-brown	Vitreous to sub-vitreous, may also be dull	STAUROLITE (Fe,Mg,Zn)2Al9(Si,Al)4O22(OH)2	Monoclinic pseudo. Orthorhombic	Usually as either short- to long-prismatic crystals, frequently twinned ("fairy crosses"), with twining at either 90o or 60o	3.74 to 3.83	Usually in intermediate grade pelitic metamorphic rocks
7 to 7½	Greyish-blue to Blue, Grey, Greenish-blue, Violet	Vitreous to oily	CORDIERITE (Beryl Group?) (Mg,Fe)2Al3[AlSi5O18] . H2O	Orthorhombic	Usually granular or massively crystalline aggregates, crystals short-prismatic	2.53 to 2.65	Largely restricted to contact metamorphic rocks and high-grade regional metamorphic rocks.
7½	Reddish-brown to Pale-pink to Deep-pink/Flesh-colored, White to Grey to Bluish-grey to Blue, Olive-green, Green, Violet, Yellow	Vitreous to sub-vitreous, may be dull	ANDALUSITE Al2SiO5	Orthorhombic	Usually as prismatic crystals with a nearly square cross-section, may be tapered in both directions from the center and exhibit a cross-like pattern on the cross section due to carbonaceous inclusions ("chiastolite")	3.13 to 3.21	Largely a mineral of low to intermediate grade metamorphic rocks, but also known from granites and granite-pegmatites
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7½	Brown, White, Yellow, Orange, Green, Blue	Adamantine, oily on fracture surfaces	ZIRCON ZrSiO4	Tetragonal	Usually as small tetragonal prisms with pyramidal terminations, may also be bipyramidal without the prism	4.6 to 4.71	Fluoresces orange-yellow, yellow and orange. This mineral may also be metamict (structurally disrupted by radiation) and then exhibits a lower H. (6 to 7) and S.G  3.9 to 4.6 range.
7½ to 8	Pale-blue to Pale-green (gem: "aquamarine") Yellow to Golden-yellow (gem: "heliodore"), Pink to light-red (gem: "morganite"), White to Tan, Emerald-green (gem: "emerald") Colorless	Vitreous	BERYL Be3Al2Si6O18	Hexagonal	Usually as stout prismatic hexagonal crystals, also granular in matrix	2.63 to 2.78	Most often found in granite-pegmatites and rhyolites, but known from other environments as well.
Hardness	Color	Luster	Name	System	Habit	SG	Notes
7½ to 8	White to Colorless	Vitreous	PHENAKITE Be2SiO4	Trigonal	Usually massive, granular, and as modified flattened rhombs, more rarely as prismatic crystals	2.97 to 3.0	Usually found with beryl in pegmatites.
7½ to 8	Dark-green to Greenish-black, more rarely Dark-yellow to Pale-yellow	Vitreous to sub-vitreous, may be dull	GAHNITE (Spinel Group) ZnAl2O4	Isometric	Usually massive, granular, also as octahedral crystals, may be rounded or sharp	4.57	Usually in high-temperature ore deposits in crystalline schists or in pegmatites.
7½ to 8	Red, Black, Brown, Blue, Green	Vitreous to sub-vitreous	SPINEL MgAl2O4	Isometric	Usually massive, granular, and as octahedral crystals, often twinned	3.56	Distinguished from gahnite by it's lower specific gravity
8½	Yellow to Emerald-green	Vitreous	CHRYSOBERYL BeAl2O4	Orthorhombic	Usually as tabular or prismatic crystals, often twinned in heart or fish-tail shapes, may also be cyclically twinned in pseudo-hexagonal crystals	3.75	May appear red or brownish-pink in incandescent light ("alexandrite").
9	Dark-grey to Light-grey to Blue-grey to Blue (sapphire), Red (ruby), and Yellow, Brown and Green (sapphire)	Vitreous to sub-vitreous and dull	CORUNDUM Al2O3	Trigonal	Usually massive, granular (emery) and as barrel-shaped prismatic hexagonal crystals	4.0	Extremely hard – can only be scratched by moissanite (silicon carbide) and diamond.

Notes:
The Tourmaline Group species can be difficult to tell apart without specific locality data.  The overlap of color and habit can make crystals without matrix very difficult to distinguish without lab tests.

Most of the Garnet Group species can be very difficult to tell apart.  Color alone is rarely – if ever – diagnostic; and S.G.s overlap too much to make them useful alone or with color.  Mineral environment information is usually needed in conjunction with S.G. and color to even begin to guess the species.  The best bet is specific knowledge of what is found at any given locality – or lab tests in the absence of that. (Far too many "visual identifications" prove to be wrong…)

   Return to Step 14     Mineral Does Not Key: Return to Step 1

---

[ Table of Contents ] [ Introduction ] [ Identification Kit ] [ Mineral Properties ] [ Environments & Associations ] [ In Conclusion ] [ The Mineral ID Key ]

---

[Previous Table]  [Next]