Obsidian | Natural Volcanic Glass



two chunks of black obsidian side by side



Obsidian Meaning and Properties

Obsidian is natural volcanic glass that's produced when magma erupts on to the earth's surface and cools so rapidly against air, water or rocks that crystals do not have time to grow.  Magma is molten rock that lies beneath the surface of the earth but once spewed out from the vent of a volcano it becomes known as lava.  Although lava initially cools fairly rapidly after first 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 fast as in hours to days but may start forming after a few weeks but will at best be microscopic.  The resulting hardened material is natural volcanic glass.  Some of the finest grade obsidian actually forms beneath the surface of the earth as the molten magma seeps into fractures in rocks that are close to the vent of the volcano.  The glass that subsequently forms will often be free from dirt, ash and other impurities.  Obsidian is never much older than about twenty million years which is relatively young when compared to the age of many of the world's rocks and minerals.

Most magma is rich in silicon dioxide which is the main constituent of volcanic glass and usually contains at least seventy per cent.  Although often referred to as a stone that's not really accurate but finding a more appropriate word can be difficult.  One respected geologist described it as a congealed liquid with impurities of rock and a limited number of microscopic crystals. 



Obsidian | Exceptionally Sharp

The way obsidian breaks is similar to glass and the correct term for this type of break is a conchoidal fracture.  This characteristic is typical of many brittle materials which lack a crystal structure.  Edges will often be curved and exceptionally sharp and when cut or broken obsidian is one of the sharpest naturally occuring materials.  For this reason it was widely used during the Stone Age because it could be crafted into knives, spear tips and arrows and more recently scalpels used by surgeons have been made from obsidian blades instead of conventional steel.  The reason for this is because they create a finer incision hence the wound heals faster and with less tissue damage and minimal scarring.  A steel scalpel has a cutting edge similar to a saw due to the metal's crystal structure but with a material that's non crystalline the edge is completely smooth hence the incision is cleaner and more precise.

A rock that does not feature any significant internal crystalline structure is not classified as a mineral and is known instead as a mineraloid.  These substances include jet, amber, moldavite and opal all of which are naturally occurring amorphous or non crystalline solids.



Types of Obsidian

The colour of obsidian is determined by the inclusions and impurities that are 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 that exhibits circular-like patterns is better known as rainbow obsidian.  The mineral hematite causes the red and brown varieties whilst inclusions of spherulite (spherical bodies) cause whitish grey blotches in a stone that's well known as snowflake obsidian.  The presence of iron and magnesium produces black obsidian which is by far the most well known variety. 



snowflake obsidian


Snowflake Obsidian. Photo: James St. John




Interestingly the spherulites in snowflake obsidian are actually quartz crystals that have formed through a process known as devitrification.  This is a process whereby atoms within the volcanic glass rearrange themselves into an orderly repeating pattern so the material changes from being a solid that's amorphous or non crystalline, into one that's crystalline.  In other words what was once glass is turning into stone.  The word devitrification comes from the Latin word vitreus meaning glassy and transparent and vitreous is a word that's widely used to describe the surface of a stone whose surface interacts with light in a similar way to glass.



More Obsidian Facts...

The name obsidian is said to have come from a printing error involving the Latin word 'obsianus' which was meant to be a reference to an Ancient Roman named Obsius who according to Pliny the Elder (Roman author, naturalist, philosopher, 23 AD-79 AD), discovered a similar volcanic rock whilst in Ethiopia. Polished black stones were used as mirrors by the Aztecs and Greeks because of their vitreous lustre and were widely traded by many ancient cultures along trade routes primarily because of their 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.

In crystal healing obsidian is considered to be extremely powerful and was widely used during the Middle Ages to drive out evil spirits and demons. It's said to cut to the heart of the matter and can be an aid to those who are grieving or struggling to overcome obsessive behaviour. Black obsidian will bring issues to the surface and can be very confrontational to the user so be sure that you're ready for a full onslaught before using it.

Different types of obsidian can be found in many locations around the world including several U.S states.  Oregon in particular has many different gem grade varieties including mahogany, red, black, rainbow and snowflake.  On Mohs scale of mineral hardness obsidian grades 5 - 5½ but being brittle means that it must be handled very carefully.



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Further Reading

Information by Wikipedia
Facts on Geology.com
An Article Worth Reading