Obsidian

Hardness: 5-5.5 (Mohs); Color: Typically dark, often black, but can be brown, gray, or rarely green/red; Luster: Vitreous (glassy); Crystal Structure: Amorphous (lacks crystalline structure); Cleavage: None, exhibits conchoidal fracture; Specific Gravity: 2.3-2.6

Obsidian forms from the rapid cooling of felsic lava (rich in silica) with very minimal crystal growth. This quick quenching prevents atoms from arranging into a crystalline structure, resulting in a glassy texture. It is typically associated with volcanic activity and can range in age from a few million years to very recent eruptions.

Historically, obsidian was highly valued for making sharp tools, weapons (arrowheads, spear points), and cutting instruments due to its conchoidal fracture producing extremely sharp edges. Today, it is used in surgical scalpels, as an aggregate in construction, and extensively in jewelry and ornamental objects. Different varieties ('snowflake', 'rainbow', 'mahogany') are particularly prized for their aesthetic qualities.

Obsidian is essentially natural glass. Despite its dark appearance, it can be translucent to transparent when cut into thin sections. The presence of tiny gas bubbles or mineral inclusions can create interesting patterns or sheens, like in 'snowflake' or 'rainbow' obsidian. It was a crucial material for prehistoric cultures around the world.

In the field, obsidian is identifiable by its dark, glassy appearance, conchoidal fracture (shell-like breaks with sharp edges), and relatively light weight compared to other dark rocks. It does not scratch easily with a fingernail but can be scratched by quartz. It is commonly found in areas of recent volcanic activity, such as the western United States (Oregon, California, Arizona), Iceland, Mexico, Italy, and parts of the Mediterranean. Look for outcrops near past or present volcanoes where lava flows have cooled rapidly.