CASSITERITE: Bolivia (14.25, 3.5)
Cassiterite has tremendous dispersive fire, much more than diamond, that is visible in properly cut pale-colored gems. This lighter-colored material is, however, very rare except in small fragments. Cassiterite is a fine gemstone – it is rather hard, and there is no cleavage problem. It is unfortunate that cuttable rough is so scarce. Cassiterites under 5 carats are not among the rarest of rare stones, but large clean gems definitely are.
Cassiterite Value
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Cassiterite Information
| Data | Value |
| Name | Cassiterite |
| Varieties | Wood Tin |
| Crystallography | Tetragonal. Crystals prismatic, pyramidal; also botryoidal, reniform with a radial fibrous structure. Twinning common. |
| Colors | Brown, brownish black, black, colorless, gray, yellowish, greenish, red. |
| Luster | Adamantine to vitreous; greasy on fracture surfaces. |
| Hardness | 6-7 |
| Density | 6.7 – 7.1; pure material 6.99 |
| Birefringence | 0.098 |
| Cleavage | Imperfect. Fracture subconchoidal to uneven. Brittle |
| Dispersion | 0.071 (nearly twice that of diamond) |
| Stone Sizes | Clean Cassiterite gems over 1 carat are quite rare. Masses occur up to several pounds in weight, but these are opaque and are sometimes cut into cabochons. Pale brown to dark brown gems up to 15 carats have been cut; slightly flawed stones up to 25 carats are known, mostly Bolivian material. Private Collection: 9.6 (brownish, Tasmania): 11.83 (brown, England); 28.16 (brownish). Smithsonian Institution (Washington, D.C): 10 (yellow-brown, Bolivia) Devonian Group (Calgary, Alberta, Canada: 14.85, 9.51 (brownish, Bolivia) |
| Luminescence | None |
| Spectral | Not diagnostic |
| Formula | SnO2 + Fe, Ta, Nb. |
| Pleochroism | Weak to strong; greenish yellow or yellow-brown/red-brown. Most visible in strongly colored crystals. |
OPTICS: o= 2.006; e = 2.097 – 2.101
Uniaxial (+); anomalously biaxial, 2V = 0-38°, usually in zoned crystals.
OCCURRENCE: Principal ore of tin; occurs in medium to high temperature veins; metasomatic deposits; granite pegmatites; rhyolites; alluvial deposits.
Alaska; Washington; California; Nevada; South Dakota; South Carolina; Virginia.
Canada; Mexico; Cornwall, England; Portugal; Japan; China; New South Wales, Australia
Araca Mine, Bolivia: source of most of the gem material known: yellow, gray, colorless and light yellowish brown to reddish brown.
Spain: gem material in yellowish to red cuttable pieces.
Erongo tinfields, Namibia: gem material.
COMMENTS: Cassiterite has tremendous dispersive fire, much more than diamond, that is visible in properly cut pale-colored gems. This lighter-colored material is, however, very rare except in small fragments. Cassiterite is a fine gemstone – it is rather hard, and there is no cleavage problem. It is unfortunate that cuttable rough is so scarce. Cassiterites under 5 carats are not among the rarest of rare stones, but large clean gems definitely are.
NAME: The Greek word for tin is kassiteros.
