As far as traditional attributed properties of gemstones, they tend to be repetitive and general (good luck, good health, etc.) or overly specific within the same general areas (wards off drunkenness, protects against a particular disease, etc.). So, you need to decide what areas you want gems to be useful in and just assign different stones (or different aspects of a stone like being cut in a certain way or having a type of flaw) to those areas. Unless you are starting in with the gem trade in your play, it's the sort of thing you can develop as you go along; just keep good notes.
To make it easy on yourself, given your scope of stated interest, gem value and color might be enough. Green for Essence, Blue for Mentalism, Red for Channeling, Yellow for Hybrid, Clear for Healing and Defense of all types, large flaws for Evil magic, for example.
