I don't think a magnifying glass would give you knoledge of gems just the ability to look at them.
Perhaps a gem ID kit that contained a listing of gems and their approximate unappraised value and a pair of special gem inspecting specs that allow you to roll lore + a modifier given depending on how pricey the specs were that allowed you to Identify the gems.
Basically i'm saying gems could be turning up unidentified on quests and stuff and require lore to ID depending on how rare they are.
Most people might be fooled into thinking one of those cheap ziose things was a diamond if someone talked slick enough i'm sure.
I used to think granite was made of diamonds when I was a kid and i'm sure lots of half orcs might think the same.
Then again I have to say, too much detail might spoil a nice simple addition such as gems as trade goods. Keep it simple wherever possible and people enjoy it more. If you make it complex as hell no one will want to make and implement it, and if they did people won't use it. they'll just go to the jem shops and trade the gems in for cash and buy trade bars.
Also keeping it simple reduces server lag.
