Jasper is a common aggregate of microgranular quartz or chalcedony, usually red thanks to iron inclusions, but can also appear yellow, brown, green, blue, or multicolored. The stones are often speckled, banded, or displaying complex patterns, like the orbicular ocean jasper. Unlike other chalcedony-based stones, jasper is always opaque. It breaks with smooth surfaces and can be polished to a vitreous luster.

The name of jasper is often used for other speckled, spotted, or marbled stone, even if they do not fit the exact definition.