Metallic Species: Gemstone Information
I have described algodonite, bornite, cobaltite, columbite, domeykite,euxenite, hematite and samarskite here together as well as individually:they have scant use in ornament but some occur in association with gem minerals and some have occasionally been fashioned.Hematite is the alpha-form of Fe2O3 and forms rhombohedral crystals in the trigonal system; they may sometimes appear pseudocubic or prismatic. The name kidney ore describes a common habit. The cherry-red to brown streak is a useful test when carefully used; the hardness is 5–6 and the SG 5.26. When appropriately lit crystals appear a lustrous black,sometimes with iridescent tarnish. Cumbria, England, has produced notable specimens and exceptional crystals have been described from Val Tavetsch, Grisons, Switzerland, and from locations in Minas Gerais,Brazil; those at Mesa Redonda and Congonhas do Campo are notable.Hematite is not normally attracted by an iron magnet though artificial forms of similar composition do respond. The material hemetine is one example. A material reported to be from Zimbabwe and to carry the name hematite garnet has an SG of 4.16 and gives a notably intense almandine spectrum; it shows a deep red on passage of a strong ray of light.
Cobaltite is CoAs and forms cubic or pyritohedral crystals of the cubic system. Ornamentally the silver-white colour with a tinge of pink may be found attractive enough to fashion. The lustre is metallic, the hardness5.5 and the SG in the range 6.0–6.3. Notable crystals are reported from Skutterud, Norway, and Tunaberg, Sweden.Other minerals with a metallic appearance though of more interest to mineral collectors, include domeykite, algodonite, bornite,columbite, samarskite and euxenite, breithauptite and pentlandite. Few,if any, gemmological tests will distinguish them and, if their identity is suspected, a museum should be consulted for a determination. Details of their composition and properties can be found in mineralogical literature.Pyrite is included in the main alphabetical sequence of this chapter as it is far more likely to be encountered. The ‘peacock ore’ variety of bornite, with an attractive surface tarnish, has been used ornamentally.Bornite is Cu5FeS4.