Rare Gem "Guarantee"
In hearthstone, there is a limit to how many packs you can buy and not get a legendary. I don't remember the exact number but I know that after a certain number of packs with no legendary, one will be in your next pack guaranteed. This makes sure your hard earned money is getting its full value in the random system of card pack opening while still giving you the possibility of getting more if you are lucky.
There should be something like this with hextech crafting and the oh so precious rare gems. After a certain number of chests, you get a 'pity' gem. It could even be as high as 75 chests and I would be okay as long as the chance to get gems regularly still exists. This ensures that after a certain point you will get what you put your hard-earned money into getting.