I was working on a project at a factory that makes gypsum wall board. The gypsum comes to the factory on ships and is placed in stockpiles. During the gypsum processing unusable material is pulled out and put in waste piles. I found this in the waste pile. It came in with the raw (unprocessed) gypsum but was unsuitable for processing into wall board material. What is it?
I'm fairly sure that these are gypsum crystals - probably formed by solution carrying dissolved calcium sulfate into to voids in the gypsum matrix. I would call it selenite. There are a number of names for different forms of gypsum crystals that you can read about on wikipedia under "Selenite" although they don't totally agree with the usage I understand.