Although usually classified as jasper, mineralogists make a distinction between jasper and rhyolite. Rhyolite, or orbicular jasper, is a name given to a highly silicified magma or tuff that contains quartz and feldspar which has crystallised into radial aggregates of needle-like crystals, forming orbicular (spherical) structures. Rhyolites form when the equivalent of granite (which forms underground) is extruded on the surface in a volcanic eruption. Obsidian is a quick-cooling rhyolite; leopardskin and ocean jaspers are slower to cool, and small spherules begin to crystallise out of the magma. A similar process occurs with snowflake obsidian, but is the result of devitrification (breaking down of the structure), not the cooling process.
I am including rhyolite as a separate section, as not all orbicular jaspers have a specific name.