A cinder is a pyroclastic material. Cinders are extrusive igneous rocks. Cinders are similar to pumice, which has so many cavities and is such low-density that it can float on water.
The following geologic
characteristics define a cinder:
- Having bubble-like cavities, called vesicles
- Measuring not less than 2.0 millimeters in at least one dimension
- Apparent specific gravity between 1.0 and 2.0
- Typical cinders are red or black in color.
Cinders have been used on track surfaces and roads to provide additional traction in winter conditions. Cinders are also popularly employed as inorganic mulch in xeriscaping, because of excellent drainage properties and resistance to erosion. In this context, they are referred to frequently with the synonym "lava rock".