Photo by Edward Allen Gems at http://edwardallengems.com/slabs-stone_canyon.php

Jasper and agates are both a type of quartz known as chalcedony. So it makes perfect sense that sometimes you can find jasper and agate melded together, like so. Agates are usually transparent or translucent, and jasper is usually opaque, but this isn’t a hard and fast rule. Sometimes it’s very difficult to tell the difference between agate and jasper, in which case people may call it chalcedony, agate jasper, or jaspagate. It is mostly found in the American Southwest and California, like other agates and chalcedonies. The one pictured was found near the San Andreas Fault.