Make gems less boring; e.g., name them after real minerals

[Mikolaj]

But the minerals would need to be of the kinds that can have any colour. Or, if not, gems would need to have hard-wired colours, which requires some decisions, e.g., whether they should be identified from the start, etc. See LambdaHack/LambdaHack#106 (but the solution needn't be the same and some variety would actually be welcome). Brainstorming is needed, but whatever implementation we agree on, it will likely be easy to do (cooperatively, in the worst case).