The expression hard as a rock loses much of its force when one considers that tale, such as is used on tender baby bottoms, is in fact a rock. Hardness among rocks ranges from tale to the diamond, hardest mineral of all. Between fall all other minerals, rated on a scale of one to ten.
The hardness scale, devised by a German physicist named Mohs and still bearing his name, is used as one of several means of identifying mineral specimens. Other qualities may be studied before identification is certain.
Most gemstones fall between five and nine on the Mohs scale. Except the diamond, of course, which stands alone as a perfect 10!