It allows you to rank-up a 4* champ from 2 to 3. Only required gold.
You first need to get the champ fully leveled up to the lower required rank, than in the upgrade part instead of using resources such as catalyst you will be able to use the gem as another option in the upgrade screen
Step 1: check your desired champ for the rank up arrow (the light blue arrow on top right of its avatar).Step 2: choose upgradeStep 3: you will see the gem on the lower right portion of the screen. Tap to useSince ur gem is silver, it is a generic one and can be used on any champ of the said star level (4 star).
That was a horrible deal to buy.