What do you mean by gem? Those coins that will give you times X of your investment? I think you need to look at exchanges and of course market cap to check the price and the total number of supplies in circulation. Most advise to check the top exchanges.

But you could also look at other exchanges though, however the problem with less known exchanges is that you might likely encounter a withdrawal issue specially if it involves huge amount (after you found the coin and then pumped hard and you sold everything to take profits.)