The value of a gemstone is not set by its prettiness alone, or otherwise there would be a lot of chunks of colored glass considered true gems. And it’s not set by its rarity alone: quartz is possibly the most common of all minerals within the earth’s crust, but not all quartz makes a gemstone.
Gems have generally been classified as “precious” or “semi-precious.” The precious division includes diamonds, rubies, emeralds and sapphire, all of which are very hard, translucent, and can be cut to display lovely facets that allow light to enter the crystal structure of the gemstone and refract within the entire crystal – that is, causing them to sparkle. Beryl is a common enough mineral, but when spruced up with chromium at just the right place in its crystal structure, it becomes green (pure beryl is colorless; other kinds of colored beryl such as aquamarine and heliodor are well known semi-precious gems). Rubies and sapphires are both varieties of the same mineral, corundum, an alumina oxide that can include various other elements in its crystalline framework that can imbue a lovely color: chrome in a corundum framework makes rubies red; and sapphires – well the blue color can be caused by vanadium and/or a very low amount of titanium and iron with a bit of crystal chemistry “magic” called “inter-valence charge transfer.” (At this point, the science involved may merit another post altogether, as it is elegant but involves terminology like ligands, heteronuclear, and other cool stuff).
All other gems are classified as “semi-precious” even though some rare and gorgeous semi-precious stones are far more costly than more mundane diamonds. There are over a hundred mineral (and rock!) varieties recognized as gems. There are also, it seems to me, at least a hundred different varieties of quartz or hydrated silica that are considered gems, including amethyst, citrine, rutilated, smoky and even more microcrystalline varieties of chalcedony, onyx, agates, jasper, and let’s not leave out tiger’s eye, for heavens sake, and don’t forget opal and obsidian…
As a geologist, it must be mentioned that many gems have a worth more than their prettiness value: some, like garnets, are essential tools in the petrologist’s arsenal of means to determine where a rock came from and at what temperature and pressure was it formed. Without zircons, it is questionable if we’d know the age of the earth. And as most know, rubies and emeralds and even good old quartz are used in laser technology.
A recommendation: to all of you who may be studying mineralogy or teaching the study of mineralogy, make sure you learn (or teach) the identification of about twelve kinds of semi-precious quartz/silica gemstones. Why? Your parents or friends won’t be able to understand a thing about crystal symmetry, or silica tetrahedrons, or inter-valence charge transference, but they all will pull out a bit of a pretty gemstone and expect you to identify it for them. If you can’t, just guess quartz, and you’ll probably be right.
The photo has been downloaded from the following site, which well deserves a visit just to see a great range of gorgeous gems:
Other great internet places where “prettiness” mixes with real science: