<http://schema.org/description>
"Gravel, small stones found in natural deposits or beds. The stones normally range in size from 14 inch (6 mm) to 2 12 inches (6 cm) in diameter, although the range sometimes is much larger."^^<http://www.w3.org/2001/XMLSchema#string>
<http://science.howstuffworks.com/dictionary/petrology-terms/gravel-info.htm>