Obsidian

WHAT IS IT?

• Obsidian is a volcanic (igneous) glass formed by rapid cooling (often due to the presence of water).

• Since it is not crystalline, obsidian is not a true mineral (it is a mineraloid)

• It is hard & brittle

• When fractured it forms extremely sharp edge making it useful for cutting tools (esp. ancient tools)

• It is felsic (high in feldspar & quartz), like muscovite, orthoclase, and plagioclase feldspars

• It is not mafic (high in magnesium & iron). Mafic materials include basalt, dolerite, and gabbro.

• It is found in the margins between rhyoltic lava flows in what are known as "obsidian flows"

• Its high silica content produces a high viscosity lava that inhibits atomic diffusion (movement) inhibiting crystal growth

• It has a low water content (<1%). When exposed to water it forms perlite, an industrial material.

• Obsidian is metastable & will crystallize over long periods of time (in a process known as ('devitrification')

• Varies from slightly opaque to transparent

• Hardness: 5 to 5.5 (much softer than quartz which has a hardness of 7)

• Lacks open voids or large bubble like other volcanic rocks

HISTORY

 The name comes from the Roman historian & philosopher Pliny the Elder (Gaius Plinius Secundus, 23-79 AD) who named it "obsian" after the Roman explorer Obsius who found a similar stone In Ethiopia

TYPES OF OBSIDIAN (partial list)

• Apache Tears — small obsidian nodules in a greyish-white perlite matrix (found in Arizona)

• Colored types — Brown, Blue, Blue-Green, Electric Blue, Deep Purple, Green, Grey, Purple, Red, Strawberry, Yellow, etc.

• Fire Obsidian — has color play inside the stone (similar to opal)

• Fireworks Obsidian — has light pink to dark red color bursts (blotches)

• Jasper Obsidian — is a mixture of obsidian and jasper (there are numerous stones that have a combination of obsidian & other materials)

• Macusanite— a green obsidian from Peru that contains floating crystals of rare virgilte (LiAlSi2O6)

• Mahogany (or Red Ribbon Black) Obsidian — has black and reddish brown colored ribbon patterns

• Sheen Obsidians: (have colored sheens due to small bubbles)

  • Gold, Green, Silver, etc. surface sheens

  • Colored obsidians (internal colors) with surface sheens

  • Rainbow— has a multicolored (Iridescent) rainbow-like surface sheen (due to bubbles). Can also have internal colors.

• Snowflake Obsidian — contains snowflake patterned blotches of cristobalite (a silica polymorph)

• Spiderweb (aka Lightning Bolt) Obsidian — is dark black with thin white veins that resemble spiderweb (from Mexico)

• Starry Night Obsidian — has white to colored spots and patterns on a black background

• Tri or Triple Flow Obsidian — has 3 colors — red, black & clear

• Velvet Peacock (aka Sea Foam or Pearl) Obsidian — has a mixture of pink, lavender, & green colors (usually from Mexico)

• Obsidian-like stones:

  • Helenite — a man-made blue obsidian gemstone made from Mount St Helens volcanic ash

  • Tachylite — basaltic volcanic glass that is much less common than obsidian. Often called tachylitic obsidian.

Warning — transparent, evenly colored blue, red, yellow or green obsidians are rare.  Most of those being marketed as natural stones are actually artificial (examples: tengizite obsidian, andara obsidian)

WHERE IS IT FOUND?

• Obsidian is found volcanic locations that have had rhyolitic eruptions

• It is found in Argentina, Armenia, Azerbaijan , Australia, Canada, Chile, Georgia, Greece, El Salvador, Guatemala, Iceland, Italy, Japan, Kenya, Mexico, New Zealand, Papua New Guinea, Peru, Scotland, Turkey, and the US (esp. Alaska, Arizona, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico, N. Carolina, Oregon, Pennsylvania, S. Dakota, Texas, Utah, Virginia & Washington)

• Oregon is the best source of obsidian n the US (esp. Bear Valley, Glass Butte & Newberry Crater)

• Areas in the US that have easy access: Inyo Crater CA, Medicine Lake CA, Newberry Crater OR, Yellowstone Park

WHAT IS IT USED FOR?

• In ancient times obsidian was used to make tools like blades, arrowheads, spear points, and mirrors.

• A method called "obsidian hydration dating" is used to determine the age of obsidian artifacts

• The Mayans used swords made of obsidian

• Obsidian scalpels have been used by some surgeons

• Use for ornamental purposes and as a gemstone

• In the 1970's, obsidian was used for the base (plinth) of high-end audio turntables

• Jewelry & decorations