Probably the rocks that contain the gems will have something that could dissolve into the aquarium water. The vinegar test is simple but limited. If you want gems in the aquarium, why not try some of those shiny things that are sold especially for fish tanks.
And now, here's an almost tale of woe. I had a big hunk of rock (petrified wood) in a 55 gallon aquarium and was messing with the plants one day when the rock crashed against the front glass. After I was all unpuckered, I checked the glass for cracks and chips, and none were apparent. It's not nice to scare an old man like that, so now the aquariums all have driftwood and no rocks. Malaysian driftwood and maybe other kinds can gradually lower pH and soften the water. The pucker factor of driftwood is also quite low.