Are there any gemstones in Mammoth Cave?

Answer 1

Probably not! I can't speak for specific caves but as a general rule caves in limestone are not gem-stone mineral sources.

It's important too, to differentiate between gem-stones and gem-stone minerals, because the former are simply high-enough quality specimens of the minerals to yield cut ornaments.

That's not to dismiss it entirely: caves formed in marble may hold gem-stone minerals (not the same as gem-stones) if the passages have cut into igneous dykes or been affected by later hydrothermal activity.

There is also the point that you never remove any of a cave's natural contents except in 3 situations: 1) "digging", i.e. clearing silt or boulder obstacles to discover where the passage leads; 2) minimum-size samples for genuine laboratory analysis; or 3) for proper preservation and display when the cave is being destroyed by quarrying or civil-engineering.

So even if the cave holds "gem" minerals... you don't touch 'em!