You can obtain Sapphire Gypsum by defeating the final bosses of The Lazarus Instrumentality and Garden of Genesis.
Cooldown
Drop Quantity Limit: 2
Time Between Drops: 2m 50s - 3m
Limit Expiration: 18h
Note: the drop counts towards the limit as soon as it is on the ground (when a loot bag drops). If you do not pick it up, you will miss one gypsum until the limit expires.