Gems were implemented by LootMarket to retain customers. You get more gems as you sell more items, and they can be used to unlock special perks around the website.
How do I get gems? Each cent that you spend, you get a gem. If you sell $10.00, you make 1000 gems.
Why do I care about gems?
1) They let you place buy orders for 500 gems. The purpose of this business is to not be involved in the transactions, just in making the processes and modelling. Therefore, buy orders are 100% required. Through this thinking, you need to resell any item you place a buy order on for at least $5.00 to break even on gems spent. You can reduce this fee with LootElite, you can read that here.
It is pretty hard to obtain through regular trading. However, this is worth noting because decreasing sales commission is a HUGE competitive advantage, and this will be explored in greater depth in future posts.
Now you understand the basic concept. Go read how to use gems to your advantage.