What do you mean find gems? and it has nothing to do with exchanges. They are just a platform which you can trade. It is still up to the buyers and sellers of a particular coin. If you really wanted to find gems then look for the coin itself and not exchanges. Of course its better if these coins are listed on a trusted exchanges, but then again, most of them are just for pump-and-dump so its really hard unless you monitor a certain coin and see if it has real usage.