I expect that someone who has visited Playacar will answer before long as I haven't been to this resort but have visited Mexico a couple of times a few years ago. I have bought some beautiful gold jewellery there in the past, mostly 14carat, the prices were really reasonable though with the current state of the pound things may not appear so cheap.
Mexico is really famous for it's silver and there is an awesome array of silver jewellery for sale.

You will struggle in Playacar as there isnt much there.You will be better of looking in Playa del Carmen (5 min cab ride) or Cancun.
Remember a few places selling gold but as the previous poster said its mainly silver.

As Aaron say you will struggle to find anything in Playacar. Just to clarify things which seem to confuse people, Playacar is an area where most of the hotels and quite a few villas are located its not a fully gated resort but access is limited, there are about a dozen or so small shops (mainly souvenir, Starbucks, a bank and kiosks with tour agents) in an area called Playacar Plaza, the nearby town is Playa del Carmen about a mile and a half away which has many shops, bars, restaurants clubs etc. It's a $5 US or 50 peso, (better to use pesos it works out cheaper) taxi ride either way at any time of day or night.
Not sure about the price and availability of gold jewelery, may be worth asking on the Playa Info forum at:-http://www.playa.info/ there are a lot of people who live in Playa, as well as a lot of tourists who regularly post on the forum there and I'm sure someone will be able to help.
We returned from Playacar a week ago after our 3rd successive year there and are already talking about returning next year.
RG