Gypsum

Like thousands of other people, I have been taking an online course called "The Fascination of Crystals and Symmetry" on Iversity.

One of the lecture videos this week shows a Gypsum crystal and tells us that its crystal class is "prismatic." Now, I know my own gypsum's crystal class thanks to the course.

a screenshot of the relevant video from the course website.

My own gypsum (I am not sure of the purity though).