Black Maltese Cross on a gem - What is it?

I have several of the same gem and see where stat values go up as the gem levels go up, but what is that Black Maltese Cross on a gem for? I have 3 Cleave gems + 2 Vaal Cleave gems. The Vall Cleaves are 1 and 8, the regular Cleaves are 1, 10 with the cross, and 12.

Also, I just bought a Gem tab and have no idea what the various levels mean other than they are definitely NOT Quality numbers.

Finally, What the heck do I do with all these gems I won't use for several years at the soonest? All I've seen so far is adding it to an Iron Ring to see the new resistance number than upgrading it to rare to see if it's any good.

The black cross just means that the gem is ready to be levelled up as it has reached the required experience.