Calcite

Calcite is an abundant mineral found worldwide.  It is the main component of many sedimentary rocks, most notably limestone and marble. Most stalactites and stalagmites are made primarily out of calcite.  

Calcite Beads

Calcite has a very low Mohs Hardness of only 3 which is softer than your fingernail. 

If you buy a rosary with Calcite beads be sure to keep it in a rosary case or in your pocket by itself.  Try to avoid putting it into a pocket with keys or loose change to avoid scratching it.  Unless that’s the look you're going for!