Personally, I like the idea of Gem venom. Particularly if it could have freaky effects on the Gems, rather than just being generically poisonous. Maybe give it a sort of hallucinatory or unbalancing effect? Have it mess with their equilibrium somehow. It would make for a good saboteur-type character. Sneaky and subtle and stuff.

...This coming from the guy whose Gem has the power of "turn into a living lightning bolt."