1. Obsidian Produced When Magma Cools Down
2. Obsidian Sharp as a Razor Blade
3. Colours of Obsidian and Curious Inclusions
4. More Obsidian Facts
5. Our Collection of Obsidian
6. More Information
Obsidian Produced When Magma Cools Down
Obsidian is natural volcanic glass that's produced as 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 being expelled, it has insulating properties so the cooling process slows down significantly over time. Crystals do not have time to grow if the lava 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 volcanic glass.
A material that does not feature any significant internal crystalline structure (no crystals present) is not classified as a mineral but is known instead as a mineraloid. These substances include jet, amber, moldavite, pearl and opal all of which are naturally occurring amorphous or non crystalline solids.
Some of the finest grade obsidian forms beneath the surface of the earth as 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 contains at least 70% silicon dioxide which is the main constituent of volcanic glass. Although often referred to as a stone, that's not quite accurate but finding a more appropriate word to describe obsidian can be difficult. One respected geologist describes the material as a congealed liquid with impurities of rock and a limited number of microscopic crystals.
Obsidian Sharp as a Razor Blade
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 can be as sharp as a razor blade and in fact obsidian is one of the sharpest of all 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. They were used because they create a finer incision hence wounds heal faster with less tissue damage and minimal scarring. A steel scalpel has a cutting edge similar to a saw due to the metal's crystalline structure but with a non crystalline material, the edge is completely smooth hence the incision is cleaner and more precise.
Colours of Obsidian and Curious Inclusions
The colour of obsidian is determined by 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 known as rainbow obsidian. The mineral hematite causes the red and brown varieties whilst inclusions known as spherulites (spherical bodies) cause whitish grey blotches in a material known as snowflake obsidian. The presence of iron and magnesium produces black obsidian which is by far the most well known.
Snowflake obsidian. Photo; James St. John
The spherulites in snowflake obsidian are difficult to see in any great detail without powerful magnification. They're known to have a radiating array of fibrous needle-like crystals typically made up of quartz and feldspar which have formed through a process known as devitrification. Devitrification 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 transforming and turning into stone.
The word devitrification comes from the Latin word vitreus meaning glassy and transparent. 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. Spherulites tend to have a duller lustre than that of the host rock.
More Obsidian Facts
The name obsidian (Latin Obsidiānus) is said to have come from a printing error involving the Latin word Obsiānus which 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 stones were also used as mirrors by the Aztecs and Greeks. Material was widely traded by many ancient cultures along trade routes 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.
Red obsidian coloured by hematite
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.
Our Collection of Obsidian