With a hardness of 3, calcite is softer than glass (5.5), so it does not leave a scratch.

What is the hardest precious stone?

A stone's Mohs rating provides a measure of its scratch resistance to other minerals. Diamond is known to be the hardest and can scratch any other stone. Talc is the softest. Reference minerals in between include gypsum, calcite, fluorite, apatite, orthoclase feldspar, quartz, topaz, and conundrum.

What is the hardest material on earth?

The outermost shell of each carbon atom has four electrons. In diamond, these electrons are shared with four other carbon atoms to form very strong chemical bonds resulting in an extremely rigid tetrahedral crystal. It is this simple, tightly-bonded arrangement that makes diamond one of the hardest substances on Earth.