Amethyst is a nice mineral that simulates Dark Stone pretty well IMO. I want to get some to glue on the bases of some of my models. Its also fairly cheap and easy to come by.
Personally, I like purple fluoride more, its a darker purple, but not sure how accessible it is.
Yeah that's nice too! Although indeed amethyst is probably much easier to get and cheaper.
And @Stormknight, if you're not careful, you can damage anything with everything