Obsidian is igneous rocks formed as a result of magma, the parent material, cooling rapidly above ground, which is the parent material. Obsidian is actually glass and not a mixture of minerals. Obsidian is the result of volcanic lava coming in contact with water. Often the lava pours into a lake or ocean and is cooled quickly. This process produces a glassy texture in the resulting rock. Iron and magnesium give the obsidian a dark green to black color. When obsidian is ejected from a volcano and cools rapidly in the air and falls to earth it is known as an obsidian bomb.
SiO2; Mostly silicon dioxide with large amounts of impurities.
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obsidian is usually dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. Obsidian consists mainly of SiO2 (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite (parent material). Because obsidian is metastable at the Earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. This breakdown of obsidian is accelerated by the presence of water. Obsidian having low water content when newly formed, typically less than 1% water by weight, becomes progressively hydrated when exposed to groundwater, forming perlite.
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark brown to black color. Very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. These bubbles can produce interesting effects such as a golden sheen (sheen obsidian) or an iridescent, rainbow-like sheen (rainbow obsidian).
Obsidian has been used by ancient people as a cutting tool, for weapons, and for ceremonial purposes and is sometimes found by archaeologists in excavations. The edges of this rock are very sharp.
- Color is dark green to dark brown and black, also can show sheens of gold or green, yellow, blue and/or purple coloration. Sometimes with white inclusions (Snowflake Obsidian).
- Luster is vitreous.
- Transparency: Obsidian is translucent in any stone of appreciable size.
- Crystal System does not apply because obsidian is amorphous.
- Habits include compact nodules or as massive layers between other volcanic rocks
- Fracture is conchoidal.
- Hardness is 5 - 5.5 (much softer than quartz).
- Specific Gravity is approximately 2.6 (average)
- Streak is white.
- Other Characteristics: Generally lacks open voids or large bubbles like other volcanic rocks.
- Notable Occurrences include Italy; Mexico; Scotland; Arizona, Colorado, Texas, Utah and Idaho, USA, as well as the Cascade Volcanic Mountain range and its associated lava beds, which stretches from Northern California into Washington state.
- Best Field Indicators are color, fracture, flow bubbles, softness, association with other volcanic rocks and lack of crystal faces.
- A volcanic bomb is a mass of molten rock larger than 64 mm (2.5 inches) in diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They cool into solid fragments before they reach the ground. Because volcanic bombs cool after they leave the volcano, they do not have grains making them extrusive igneous rocks. Volcanic bombs can be thrown many kilometres from an erupting vent, and often acquire aerodynamic shapes during their flight.