Fluorite was my favorite mineral when I was growing up. I have a tough time picking favorites, so this choice is something very meaningful for me. Fluorite has a Mohs hardness of 4, which isn't very strong (diamond is a 10!). But sometimes you can still appreciate things that are weak and easily hurt, like me. The sample of fluorite pictured here is purple, but fluorite comes in many colors such as blue, green, and yellow. This a fact you would know if you visited the collection in my childhood bedroom.
If you want to learn about the Mohs hardness scale and other methods for mechanical testing of materials you can watch my video on chairs.