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Obsidian is a mineral-like, but not a true mineral because (as a glass) it isn’t crystalline. It is a congealed liquid (with almost total absence of sizable mineral crystals), thus it’s amorphous.
It is a natural glass that was originally molten magma. It formed when lava - extruded from a volcano - cools quickly above ground through the glass transition temperature and freezes without sufficient time for crystal growth.
When I say "crystals," don't imagine those beautiful and precious stones. All rocks consist of mixtures of various crystalline minerals. When crystallization occurs, the atoms become arranged in regular and geometric patterns that are unique to the specific mineral. Crystal surfaces forms only where there is enough open space in the rock mass to allow the natural geometric forms of the crystals to develop as free surfaces.
The processes of plate tectonics create geologic conditions favourable to volcanism and the formation of obsidian.
Obsidian occurs only (in or near areas of crustal instability) where:
- geologic processes create volcanoes
- the chemical composition of the magma is rich in silica (Obsidian consists of about 70 percent non-crystallized silica)
Obsidian is relatively young in comparison to most continental rocks that form the Earth's crust. Over a long period of time, obsidian gradually changes from glass to rock. In this process, the silica molecules slowly rearrange into organized crystal patterns.

Obsidian composition is similar to granite, but cooling rate and water content of the magma, make the difference. Granite cools very slowly miles below the surface of the earth; this allows for the formation of sizable mineral crystals.

When magma approaches the earth's surface and the pressure decreases, most of the water in the magma is lost as steam.
The resulting silica-rich magma with little remaining water becomes very viscous obsidian magma. This magma is so viscous that sizable mineral crystals cannot grow. Silica-rich magma squeezes into rock fractures to form layers of obsidian that are relatively free of impurities.
Clear varieties of obsidian contain very few opaque impurities or microscopic mineral crystals.
A certain amount of water always is present in obsidian. Very small inclusions of water steam in the form of bubbles often are trapped in the glass.
These bubbles can produce interesting effects such as sheen and some of these bubbles are visible to the naked eye. Tiny gas bubbles cause the reflectance of obsidian.
The high viscosity of the molten obsidian prevents effective mixing of these magmas, resulting in obsidian that is "streaked" with different layers or colours. Each of these streaks or lines may represent a distinct obsidian eruption.

Some obsidian is erupted as lava flows at the ground surface; these surface flows are so viscous that they flow very slowly.
However, the best quality obsidian often forms below the ground surface around volcanic vents.

Obsidian is translucent and it has similar properties to quartz because of a similar chemistry. However, obsidian lacks any crystal structure. Because of the lack of crystal structure, obsidian blade edges are very sharp and they can reach almost molecular thinness, leading to its ancient use (as a cutting tool, for weapons and for ceremonial purposes) and its modern use as surgical scalpel blades.
Pure obsidian is usually dark in appearance, though the colour varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to dark brown and black colour. It also can show sheens of gold or green, yellow, blue and/or purple coloration. Sometimes with white inclusions.
Each volcano (and in some cases each volcanic eruption) produces a distinguishable type of obsidian, making possible for archaeologists to trace the origins of a particular artefact.
Modern archaeologists have developed a dating system Obsidian hydration dating to calculate the age of Obsidian artefacts. This measures the microscopic amount of water absorbed on freshly broken surfaces.
Obsidian breaks with a characteristic "conchoidal" fracture. This smooth, curved type of fracture surface occurs because of the near-absence of mineral crystals in the glass. The intersections of conchoidal fracture surfaces can be sharper than a razor.

Historical use
Obsidian was highly valued in certain Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. It was also polished to create early mirrors.
Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated, including carved and worked obsidian for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. The weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon.

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