Obsidian is a naturally occurring volcanic glass. It has minimal crystal growth as it is a glass and as such can be used to make blades with very thin edges. The most common color of obsidian rocks is black; however it can also be found in brown and green and more rarely blue, red, orange or yellow. The colors are thought to be caused by other elements being present in the cooling process. It has been used to make cutting tools for thousands of years and is still used in making scalpels today.
Pumice is a rock formed from frothy lava. It has many air bubbles in it, which can be due the presence of water in the lava, causing rapid cooling with rapid depressurization which creates the bubbles. Dry pumice can in fact float on water. The most common colors of pumice are tan and light gray but it can appear in other colors, such as white or black. It is used in the construction of breeze blocks and concrete, as an abrasive in items like polishes, and as a decorative stone.
Rhyolite is a light-colored, fine-grained, extrusive igneous rock. It contains quartz and feldspar minerals. It can often contain some gas bubbles frozen into the rock. It is usually light colored: light gray, tan, reddish, greenish or brown. Although it is fine grained you can sometimes find larger crystals such as zircons embedded in the rock. Some types of rhyolite such as "Wonderstone" and "Hickoryite" are sought after as gem rocks. The fine grain in rhyolite also makes it an attractive rock to use in construction to incorporate colorful, pleasing accents.
Basalt is a hard-stone igneous rock and one of the most common rock types in the world. It is formed by the slow cooling of lava with few air bubbles present. It has a variety of uses worldwide--for example in cobblestones and making statues. The strength of basalt makes it ideal for this purpose as it is very difficult to crush. Heating and extruding basalt allows the formation of "stone wool" for thermal insulation.
