Corundumite

Corundumites are flying earth elementals that look like faceted red or blue gemstones. Drawn to veins rich with rubies, sapphires, and other precious stones, corundumites regard themselves as the guardians of the earth's riches, attacking those who mine or even examine the uncut gems in their domains. They can be hostile to travelers who have gemstones in their possession, but those who carry no such stones—or who surrender their gems as recompense for the slight—are usually allowed to pass unharmed. Why corundumites are so protective of the stones in their care remains unknown, though those who have spoken with the elementals report that they seem to regard the gems not as symbols of wealth, but rather as living, thinking beings.

Gem Body

When it dies, a corundumite transforms into an inert—and highly valuable—corundum gemstone such as a ruby or sapphire. An intact gemstone harvested from a dead corundumite can be worth as much as 2,000 gp, provided it's in good condition. Unfortunately, not every battle leaves behind a perfectly preserved gemstone. If the damage that reduced the corundumite to 0 Hit Points was bludgeoning, force, or sonic damage, the gem smashes into 2d6 fragments, each of which is worth only 100 gp.