Obsidian is cooled quickly, thus there is no time for crystals to form and the result is volcanic glass. It is still used today to make special, extremely sharp scalpels used for eye surgery. Rhyolite and basalt result from magma which cooled slowly enough for crystals to form. The slower the cooling process, the larger the crystals become. Lava also exhibits various shape characteristics once it cools.
Pahoehoe refers to lava which has a ropey, plastic, wrinkled appearance.
Extremely fine volcanic ash can be propelled into the upper atmosphere where it can remain for months or years, circling the earth. Larger ash, like sand, falls back to the ground and can do great damage. A layer of ash a few inches thick is so heavy it can crush the roof of an average home. The terms ash, lapilli, and bomb refer to the sizes of volcanic material ejected. The consistency of this material is described by additional terms such as pumice which is a frothy material which contains so much trapped gas that it floats, scoria which is heavier than water, and spatter which refers to material which is still soft when it hits the ground.