So we have four types of gems: 100gp, 500gp, 1000gp, and 5000gp gems. But are we really supposed to believe that a few isolated settlements separated by expanses full of hideous monsters have a standardized system for dealing with the value of gemstones? Although the PHB doesn't explicitly say, I am assuming that Streetwise is the skill used for appraising and haggling. Are there explicit rules somewhere for determining how much you can sell a gemstore for in a given city?