The chemical formula of Obsidian is indicated by SiO2, mostly Silicon Dioxide with large amounts of impurities. Obsidian is actually a Mineraloid. It is used as a semiprecious stone and ornamental stone for carvings. Obsidian is actually a result of volcanic lava coming in contact with water. As the lava pours into a lake or ocean, it cooled off so quickly. This process produces a glassy texture in the resulting rock. The dark green to black color of Obsidian is brought about by the presence of iron and magnesium in its chemistry. Ancient people used Obsidian as cutting tool, for weapons, and for ceremonial purposes. It is also sometimes found by the archeologists in excavations.
Obsidian is most commonly found dark green to dark brown and black in color. It can also show sheens of gold or green, yellow, blue and/or purple coloration which can be more splendidly exhibited when viewed with the aid of petrographic polarizing microscopes. Sometimes, Obsidian can be found with white inclusions like the Snowflake Obsidian. Obsidian is usually found showing vitreous luster when viewed in reflected light of polarized light microscopes used in optical mineralogy. Most Obsidian crystals are found translucent in appearance in any appreciable size. Crystal system does not apply since Obsidian is amorphous. The crystal habit of Obsidian as described in the field of optical mineralogy includes compact nodules or as massive layers between other volcanic rocks.
Obsidian has several varieties known in optical mineralogy. It can actually contain small bubbles of air that are aligned along layers created as the molten rock was flowing just before being cooled. These bubbles can actually produce interesting effects such as golden sheen. This variety is known as Sheen Obsidian or a rainbow sheen called the Rainbow Obsidian. Inclusions of small, white, radially clustered crystals of cristobalite in the black glass can actually produce a blotchy or snowflake pattern producing Snowflake Obsidian. Another variety is the small nuggets of obsidian that have been naturally rounded and smoothed by wind and water and this is called Apache Tears.
Obsidian is most commonly found showing conchoidal fracture when viewed with the aid of polarized microscopes used in optical mineralogy. The hardness measure of the mineral when it is evaluated using the Mohs scale method is usually ranging from 5 to 5.5, which is much softer than quartz. The specific gravity measure of Obsidian is approximately 2.6 grams per cubic centimeters, which is considered average. Obsidian is most commonly found leaving a white streak when rubbed on a white porcelain streak plate. Obsidian generally lacks open voids or large bubbles like other volcanic rocks.
Obsidian is often confused with smoky quartz. It has similar properties to quartz because of a similar chemistry. However, many properties dependent on a crystal structure are altered or absent in Obsidian because it lacks any crystal structure of its own. Therefore, Obsidian also lacks the piezoelectric and optical properties of quartz. Smoky quartz usually has a splotchy or zoned distribution to its color while that of the Obsidian is more uniformly distributed. The best field indicators of Obsidian include fracture, color, softness, flow bubbles, lack of crystal faces, and association with other volcanic rocks. Obsidian notably occurs at its type of localities including Italy, Mexico, some areas in USA, and other associated lava beds.
