The Mohs Scale of Hardness measures a mineral's resistance to being scratched. The Mohs scale ranges from 1 to 10, with talc rating lowest (1) and diamond rating highest (10). That means that almost every mineral is harder than (or "capable of scratching") talc, but no other mineral is harder than diamond. The scale was invented by Austrian mineralogist Friedrich Mohs (1773-1839).
The gem trade standard for hardness. The higher the Mohs Scale number, the harder the stone. The highest Mohs Scale rating is 10 (diamonds). Any piece of jewelry with a Mohs Scale rating of less than 7 is easily scratched (coral, opal, pearl, turquoise).