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Obsidian Properties Facts and Photos

two pieces of natural black obsidian side by side isolated on a white background

1. Obsidian Natural Volcanic Glass
2. Sharp as a Razor Blade
3. Colours and Inclusions
4. More Facts
5. Our Collection of Obsidian

Obsidian Natural Volcanic Glass

Obsidian is natural volcanic glass produced as magma erupts onto the Earth's surface.  This burning hot material then cools so rapidly against air water or rocks that crystals do not have time to grow.

Magma is molten rock from beneath Earth's surface.  Once spewed out from the vent of a volcano it becomes known as lava.

Although lava initially cools fairly rapidly after being expelled, it has insulating properties so the cooling process slows down significantly over time.

Crystals do not have time to grow if it cools very fast as in hours to days.  They may start forming after a few weeks but will at best be microscopic.

The resulting hardened material is volcanic glass also known as obsidian.

A material that's not crystalline (no crystals present) is not classified as a mineral.  These naturally occurring substances are known instead as mineraloids.  Examples include jet, amber, moldavite, pearl and opal.

The word amorphous is also used to describe a naturally occurring non crystalline solid.

Some of the finest obsidian forms beneath the surface of the earth as molten magma seeps into fractures in rocks.  These tend to be close to the vent of the volcano.  The glass that subsequently forms will often be free from dirt, ash and other impurities.

Obsidian is rarely older than about twenty million years.  When compared to the age of many rocks and minerals that's relatively young.

Most magma contains at least 70% silicon dioxide which is the main constituent of obsidian.

Although often referred to as a stone that's not quite accurate but finding a more appropriate word for obsidian can be difficult.  One respected geologist described it as "a congealed liquid with impurities of rock and a limited number of microscopic crystals". 

Sharp as a Razor Blade

The way obsidian breaks is similar to glass.  The correct term for this type of break is a conchoidal fracture.

This characteristic is typical of many brittle materials which are not crystalline.  Edges will often be curved and can be as sharp as a razor blade.
Obsidian is one of the sharpest of all naturally occuring materials.  For this reason it was widely used during the Stone Age for knives, spear tips and arrows.
More recently scalpels used by surgeons have been made from obsidian blades instead of conventional steel.  They create a finer incision hence wounds heal faster with less tissue damage.  The result is minimal scarring.

A steel scalpel has a cutting edge similar to a saw due to the metal's crystalline structure.  The edge of a non crystalline material is completely smooth so the incision is cleaner and more precise.

Colours and Inclusions

The colour of obsidian is determined by the inclusions and impurities present.  Tiny bubbles caused by water vapour, air or gas can also produce different types of sheen.

A golden sheen is known as sheen obsidian whilst iridescent material with circular-like patterns is known as rainbow obsidian.  Hematite causes the red and brown varieties whilst inclusions known as spherulites (spherical bodies) cause the whitish grey markings in snowflake obsidian.

The presence of iron and magnesium produces black obsidian which is the most well known.

snowflake obsidian mineraloid

Snowflake obsidian. Photo; James St. John

The spherulites in snowflake obsidian are difficult to see in detail without powerful magnification.  They're known to have radiating fibrous needle-like crystals.  These tend to be made up of quartz and feldspar and have formed through a process known as devitrification.
During devitrification atoms within the volcanic glass rearrange themselves into an orderly repeating pattern.  The material therefore changes from being a solid that's amorphous into one that's crystalline.  In other words what was once glass has transformed into stone.

The word devitrification comes from the Latin word vitreus meaning glassy and transparent.

Vitreous is widely used to describe the surface of a stone whose surface interacts with light in a similar way to glass.  Spherulites tend to have a duller lustre than the host rock. 

A Few More Facts

The name obsidian (Latin obsidiānus) is said to have come from a printing error involving the Latin word Obsiānus.  This was a reference to an ancient Roman named Obsius.

Pliny the Elder Roman author, naturalist and philosopher [23 AD - 79 AD] stated in his works Naturalis Historia in reference to obsian glass and obsian stone;

Among the various kinds of glass, we may also reckon Obsian glass, a substance very similar to the stone which Obsius discovered in Ethiopia.  The stone is of a very dark colour, and sometimes transparent; but it is dull to the sight, and reflects when attached as a mirror to walls, the shadow of the object rather than the image.  Many persons use it.   (Naturalis Historia chapter 67).     

Polished black obsidian was used as mirrors by the Aztecs and Greeks.  It was widely traded by many ancient cultures along the main trade routes.  This was primarily because of its suitability for crafting into blades and tools.

Obsidian continued to be used in the ancient Middle East for thousands of years after the introduction of metals.

piece of red obsidian volcanic glass isolated on a white background

Red obsidian coloured by hematite

When used for its metaphysical properties obsidian is an extremely powerful stone.  During the Middle Ages it was used to drive out evil spirits and demons.

It's said to cut to the heart of the matter and can aid those who are grieving or struggling to overcome obsessive behaviour.

Black obsidian brings issues to the surface and can be very confrontational to the user.  Those using it should be prepared for a full onslaught.

Different types of obsidian can be found in many locations around the world.  Oregon has many gem grade varieties including mahogany, red, black, rainbow and snowflake.

On Mohs scale of mineral hardness obsidian grades 5 to 5.5.  Being natural glass it's brittle so must be handled carefully.

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