By the rules, "metal" has a hardness of 10, and "gems" have a hardness of 8. Obviously, some metals (gold) are softer, and some gems (pearls) are also softer. If you wanted a simple, rules-based explanation, use hardness 8 for gems. If you want to use Moh's table, won't you have to find one for metals, too? BTW, do masterwork weapons have greater hardness or hp?