The basics
So what is calcite, really?
Calcite is calcium carbonate, one of the most common minerals on Earth and the main ingredient in limestone and marble.
It's soft at Mohs 3, so a copper coin can scratch it. A drop of acid makes it fizz, which is how geologists spot it.
Clear optical calcite, also called Iceland spar, splits light in two. Set it on a printed word and you'll see the word twice.


