Diamond
Written by Boss Tambang Tuesday, 01 December 2009 23:59
Diamond is the hardest mineral, number 10 in the Mohs scale of mineral hardness. This 4-millimeter specimen shows several faces of the natural crystal form, which is an octahedron—imagine two pyramids joined base to base so their tips point opposite directions. The flat triangles on the diamond correspond to the faces of the octahedron. Crystallographers call them (111) faces, and they are the hardest part of the diamond. Diamond crystals usually have rounded, grooved edges as can be seen next to the flat faces.
The images show diamond’s adamantine luster, the distinctive brilliant greasy surface appearance that, along with its extreme hardness, distinguishes diamond from all other minerals. Only quartz resembles it, but that mineral is softer and has a glassy luster. Diamonds form at great depth, well below the crust. They reach the surface only in peculiar eruptions that explode into the air, leaving behind bottomless pipes of mantle rocks called kimberlites. See this article for more about their geology.
I think this stone may be a twinned crystal. The two pictures show opposite sides, and the knob at the lower right of the left image is on the upper left of the right image. See if you can picture its three-dimensional shape. (For a better example of twinning, see staurolite.) Diamonds occur in a range of forms, from nearly perfect crystals to rough grains like this one to warty black lumps known as bort. All forms of diamond are valuable, either as gems or as superhard grinding and cutting grit. Like most diamonds mined today this is African, mined in the Tshikapa gravel beds in Kasai province of the Democratic Republic of Congo (formerly Zaire), near the Angolan border. It was cheap enough that I might actually use it to scratch other minerals.
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