Mantle gems include diamonds and peridots. These form under intense heat and pressure many miles below our feet.

Another type of deep-Earth gem forms closer to the surface, where metamorphism takes place .

Those garnets look a lot like the ones I used to search for in the Berkshire Hills of western Massachusetts, where I grew up (and where I once stumbled across “gold“). Rocks there are layered like lasagna.

Most garnets there and elsewhere are industrial grade–deep red clear to opaque crystals that wouldn’t make you rich when collected for fun but are profitable when mined in bulk for industrial use as abrasives.

Garnets can come in every color, including blue, which was once thought to be impossible. Garnet translucency is such that even a small amount of a minor element can make a big difference.

Metamorphic gems like garnet don’t look expensive until they have passed through a gemcutter’s hands.

Green garnets are the most valuable:

• Russian tsars liked demantoid garnet, which can outsparkle both diamond and emerald.
• Tsavorite garnets rival emeralds in color and usually have fewer inclusions, but they are usually small and common. Only large stones (2 carats or more) can command super-high prices.

Even uncut tsavorite can be beautiful. (Rob Lavinsky, CC BY-SA 3.0)

Technically, garnets are a mineral group, not a specific gemstone. And that is not surprising, given the wide assortment of mineral-forming elements there are underground and all the high-pressure, high-heat geologic drama that accompanies garnet formation!

Thanks to plate tectonics and subsequent weathering, which uncovers underground rocks, garnets occur literally everywhere from Afghanistan to Zambia.

Just keep an eye on the blue ones–they’re tricky.