So the other post said to make a brilliant glass, and if it sucks, sell it for more than the cheaper gem...but what i do is totally different. Make your own brilliant glass, then if the gem sucks, you keep it in the glass overnight. Then, you take this glass and make someone elses glass. At this point you take the cheaper glass you know is Terrible , and give it to the person you made it for. So basically
1.Make your own glass
2. If its bad, make someone elses glass, and give them your shitty one.
3. And if you get a bad one in theirs, keep rinse and repeating till you get a epic gem.
* Also, if you just so happen to "Accidentally open the glass" and it puts it in the inventory, just go get the "Gift wrap" things that vendors sell, and wrap it. Then when you make someones glass just open it saying you accidentally opened it and didnt mean to. But still keep their glass for this cycle.
This way, down the line for the cost of one glass, and the money of other peoples glass, you will make one epic gem eventually. All you have to do is start the cycle.