Gosh-Garnet! It’s Time for January’s Birthstone.

Now this is a stone that a biologist can really love: the name “garnet” does not refer to just one stone, but to a series of several species (yes! Species! I told you biologists would love it) of stone, each one similar, and yet unique, in their formation and chemical structure. Oh January, January, you truly have a mess of riches.

Check out the color gradient in this garnet! Photo uploaded to wiki by Alkivar.

While it’s possible to find garnets in almost any color, the most common, well-known, and garnety-garnet is the deep red one. In fact, that vibrant color is where the name “garnet” comes from, and many people have commented on the similarity between unpolished natural garnets and pomegranate seeds. In color, shape, and size, I mean. After all, garnet is severely lacking in endosperm, would crack teeth if eaten, and planting a whole row of them would just lead to disappointment.

But the multiple species of garnet make it more than just a pretty stone: they can serve as almost little time-capsules or records of the formation and history of the rock that surrounds them. The science of interpreting the stories that these gems hold is called geothermalbarometry. Because different temperatures and pressures are necessary to create, say, a pyrope garnet rather than a spessartine garnet, the presence of these specific species of garnet in a rock can provide information about how that rock came to be. Was it pushed up from 100 KM? Was it made in the mantle? What kind of metamorphic rock are we dealing with exactly?

Garnets are hard rocks (7-7.5 usually on the Mohs scale) and so they are great for jewelry or industrial purposes, like sanding things down and abrasing other things.  If you manage to smash one apart, you won’t see nice neat cleavage (think of the thin sheets of mica that just separate out ““ now that’s good cleavage!), but sharp jagged edges.

See! Look how wild this molecular model is! By Peter Murray-Rust found in wikicommons.

This is the part of the story where I talk about the chemical composition, but with garnets, it is a bit of a different ball game. No, not a whole different ballgame. Just a little bit. All garnets are neosillicates, which means that they all share having silicon and oxygen present in the chemical formula. But then, there’s a lot of wiggle room ““ two other elements are involved, and they usually include combinations of calcium, magnesium, or  iron with aluminum, iron, or chromium. There’s a helpful image from Wikipedia that shows what that might look like. Yeah, a lot more complicated than the cubic all-carbon structure of a diamond.

So there you have it, a brief overview of the time-recording, color-tastic, old-timey pomegranate-garnet gem. I gotta say, January babies, you have it pretty good.

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