A friend asked for help categorizing rocks as gravel, pebble, or boulders. She knew pebbles were smaller than boulders, of course, but what she didn't know was whether there were actual size classifications for solid mineral matter. Once we got digging into the topic, we learned that although grade scales do exist, petrologists do not consider numerical demarcation to be (you should pardon the pun) written in stone. Let's take a look.

As far as the layperson is concerned, a rock, "an often jagged fragment of solid mineral matter," may range in size from a boulder to a pebble. A pebble is "a small, usually rounded stone, especially one worn by the action of water," while a boulder names "a detached and rounded or much-worn mass of rock."

To rock scholars (petrologists in more formal terms), quantifying and naming pebbles and boulders is a matter of choosing a particular scale. Depending on the scale chosen, the size of the finest particle, clay, can range from one-one-thousandth of a millimeter in diameter to seven-one-thousandths of a millimeter in diameter. On most scales, clay is followed in size by silt and sand. Other grades, which vary from scale to scale, include granules, gravel, pebbles, cobbles, and boulders.

Well, if you don't make me laugh, you're educating me! Excuse the "stoney silence". I was paying attention.

Methinks he's growing "boulder" every day!