Gypsum

is a resource that is found by doing a variety of activities. Gypsum can be crafted into Gypsum Orbs, which can then be turned into special caches called Gypsum Casts. Gypsum Casts are gear type specific, and when opened, guarantee an Expertise bump for that type of gear as well as an item of that type.

Details
You need a full days worth of gypsum to create a single orb. There is a single exception to this: only 1 Sapphire Gypsum can be used each day. You may be able to stockpile these for future crafts. Gypsum is not collected from the reward containers for breaches, outpost rush, arenas or trade skill aptitude level-ups. You will need to complete the activity in order to receive it as a reward. Each orb will be used to level the Expertise of an item slot and generate an item within the maximum range of that Expertise level. It is approximately 31 Orbs to level a slot from 500 Gear Score to maximum. When everything is at maximum, they will simply be used to generate items of a chosen slot with a guaranteed GS range of 591-600.

You will only be able to level up the expertise of each item slot once per day. When you are approaching maximum expertise across all item slots, players will need to do less and less gypsum generating activities per day.

Be sure to also check out this article from our friends at NWDB.info: Gypsum Farming Guide