We have availability of goods based on price and market class, so a class 3 market can buy/sell 2x 60g plate armor.
Does the same thing apply to gems? For instance, if the players want to sell a 500g topaz (or buy a topaz for 500g).
If you were to ask me whether gems were “goods” or “services”, I’d go with “goods”.
Not an official answer, but I see no rules reason, in-world reason, or gameplay reason why they’d be distinct.