You are able to make thunderbolts by using your dagger on regular bolts and then clicking on black pearls. The same goes for regular arrows + spider silk and regular arrows + sulfurous ash
Do you get rubies/diamonds/star sapphires from mining? You do from stealing at least I think?
When I was hunting earlier I got quite a few too, so 1:1 might be okay I think. It’s not like ppl are gonna burn through them for sparring like regular bolts/arrows 🙂
You will be able to get some gems from mining, but the majority will come from monsters and skinning. For example skinning a radiant wisp will give you 10 star sapphires.