This artist is into painting rocks, but she doesn't have ready access to a lot of rocks so she decided to make her own. Coloring the rocks with different paints and dyes she could create a lot of different effects. As for the clean up? She said she scrunched up the mat and the plaster popped right off!
I recognize those “molds”. They are silicone food savers. And it looks like an avocado set is good too. I found them here
https://www.thegrommet.com/products/set-of-four-assorted-colors-1
Could you please tell me your recipe for rocks and where did you get these molds that you used?
Very interested.