what did the bugs eat?
they were living down there for thousands of years. what was their food source?
shareMaybe each other? ;-)
shareIt can't just be each other. You need something lower on the food chain.
An abandoned salt mine huh? Salt mines aren't fertile growing places for subeterranean lichens and whatnot.
Beetles are very adaptable. Maybe they have some symbiotic algea or bacteria that lets them digest some otherwise indigestable material. They guard the emeralds. Maybe they have unusual digestive acid that breaks down carbon crystals, eg precious stones, and this allows the bacteria to metabolise the untapped nutrients thereof. Then the bugs consume the bacteria, or some organic biproduct thereof.