I've never been that enthusiastic about diamonds; I like a lot of vulgar color. So here's a semi-precious stone I can really get behind. Tourmaline is a silicate crystal that exhibits all kinds of colors depending on the traces of minerals that slip in. Not only does this result in an entertaining array of colors, it yields mind-boggling chemical formulae, like this for elbaite, a variety of tourmaline associated with the island of Elba: Na(Li1.5,Al1.5)Al6Si6O18(BO3)3(OH)4.
The site Grim directed us to the other day, TYWKIWDBI (which they pronounce "TeaWikiWidBee"), has a beautiful picture of tourmaline today. I won't reproduce it here, because you should go over and check out the site, but here is a fine picture from Wikipedia, showing the reddish/greenish stripes that give "watermelon" tourmaline its name:
Some more pictures from Glendale Community College:
Tourmaline crystals are prismatic and columnar crystals that are usually triangular in cross-section. The style of termination at the ends of crystals is asymmetrical, so that a typical columnar crystal resembles a pencil, long and skinny with one flat end and one pointy end. This asymmetry is called hemimorphism, and can result either from changes in crystallization conditions during the growth, or, as in the case of tourmaline, from alternating patterns inherent in the complicated crystal structure. Hemimorphic crystals tend to display an unusual level of both pyroelectricity and piezoelectricity, meaning that if you heat or squeeze the crystal, the lattice does not expand or contract. 
Instead, ions are displaced, which generates electrical potentials on the crystal faces. This effect occurs in a lot of crystals, some of which will generate a visible spark when crushed, but for some reason it is accentuated in asymmetrical crystals.
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