3.0 on the Mohs scale.

Worldwide.

Transparent and colorless. Iceland spar shows off calcite's birefractiveness very well. Iceland spar is used in the optics industry for its transparency and ability to split images. It can be used to determine the distance to an object viewed through it, and was used in the bombsights of world war II bombers. Because of calcite's softness, it should be used in jewelry only where it won't be subject to abuse, and great care should be taken with any calcite specimen. It is generally faceted only for collectors.

Calcite is not enhanced.

More information on gemstone enhancements.