What happens to my legendary gem after an ancient augmentation?

In Diablo 3 you can augment your ancient legendary items. Apparently you can augment each item multiple times and it just gets overwritten by the newer one.

My question is: What happens to the legendary gem I use for the augmentation?

Is it consumed in the process? If yes does it drop again or will it be lost forever?

Solution 1:

The legendary gem is consumed.

Legendary gems will continue to drop in Grifts as long as you don't have a copy of that gem on your active character, their companion, or in your stash.

Note that the list above is exhaustive. If a gem is on another of your characters or on the ground, it can drop in a GRift.