kw: minerals, gemstones
I was recently sorting some jasper material to determine what to donate to an educational rock club and what to keep "in case I want it later". Jasper is an opaque, semiprecious material that is cut into cabochons, never faceted into "gemstones". But it got me thinking about the hobbyists I know that do like the clear stones and make faceted stones.
Firstly, a little looking around confirmed that the gems termed "precious" are all the really hard ones: diamond, sapphire, ruby and emerald. Topaz is sometimes included, but most professionals prefer to group it with quartz and moonstone (a feldspar) as "semiprecious". Jasper is a quartz variation. There is no such thing as a "nonprecious" gem!
Hardness is the first quality that makes for a desirable gemstone, particularly if you plan to wear it. A hard mineral won't scratch easily (and only diamond can scratch a diamond). A subtler matter is that the harder a stone is the better it can be polished. Under a microscope, the best "paste" (glass) or "flint paste" (quartz) polished stone will be seen to have a trace of roughness, which limits the amount of light that can enter and leave the stone. The extra-smooth polish that only a diamond can acquire contributes quite a lot to the "adamantine" luster of a properly cut diamond.
Anyone who has taken an Earth Science or Geology course has learned the Mohs scale of hardness. Ten well-known stones were ranked for hardness, and a set of nine of these is used to make a "scratch tester". The vendors of inexpensive scratch test kits can be forgiven for not including a piece of diamond! They also usually substitute silicon carbide or tungsten carbide for #9. These artificial materials are much less costly than even a small hunk of clear-ish corundum. The scale is not in any way linear, but simply a ranked list.
Various methods have been devised for determining absolute hardness. About a century ago, things settled down, and a test that is not too hard to perform in the laboratory is the Knoop Indentation Test. A small piece of industrial diamond, ground to a sharp point, is pressed against a material using a known weight, for several seconds. The length of the impression the diamond makes is measured and converted to a number. The result for the ten Mohs materials is shown on this chart, which has a logarithmic scale:
You can see it is not at all linear. However, on this scale, the minerals numbered 5 through 9 are pretty close to a linear set. If they are taken as the determiners of a near-linear scale, numbers 3 and 4 would be close together near 2, and the two softest materials would be "below zero". Diamond would be about 14. It is about four times as hard as corundum, and 3½ times as hard as silicon carbide or tungsten carbide.
A recent innovation in the scale is being proposed using "fullerite", which is claimed to be harder than diamond, as the indenting material. I expect this to be resolved within a few years. Meanwhile, the hard materials make the best gemstones, not only because they are hard, but because they take the best polish and are thus brighter. The addition of clear silicon carbide, called Moissanite when in gemstone form, to the gemstone universe adds interest, and it remains to be seen if it becomes a coveted material.