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 colder rocks that crystals do not have time to form. Magma lies beneath the earth's surface and is hot enough to melt rocks, the deeper the magma, the hotter it becomes. Once it reaches the earth's surface it becomes known as lava but the only thing that has really changed, is the name. Burning hot magma is initially white but as it cools it turns yellow and then various shades of red. Once it cools sufficiently enough to enable it to solidify it becomes an igneous rock. If the magma cools down slowly say between a couple of thousand to a few million years and is beneath the earth's surface, crystals have time to grow but when it cools quickly whilst above the surface as in days to weeks, they do not have time to grow hence they're microscopic in size. If cooling is very fast as in hours to days, no crystals form and the result is volcanic glass or obsidian. A rock that lacks any proper crystal structure is not a mineral in the true sense and is instead correctly known as a mineraloid.
Iron and magnesium cause the dark colouration in the black variety of obsidian but the colour can change depending on what inclusions or impurities are present. Tiny bubbles of air or gas can change the appearance of the the stone by producing different types of sheen. Obsidian that exhibits a golden sheen is known as sheen obsidian whilst those that exhibit an iridescent rainbow-like sheen are known as rainbow obsidian. The presence of the mineral hematite produces red and brown varieties of obsidian and when inclusions of spherulite are present it causes blotchy snowflake-like patterns, these stones are known as snowflake obsidian.
Obsidian when cut or broken is one of the sharpest materials found in nature and the way in which it breaks is characteristic of glass and other brittle materials. For this reason it was used extensively during the Stone Age because it could be crafted into knives, spear tips and arrows and a well crafted obsidian blade can be many times sharper than a surgical scalpel. It has been reported that a small number of surgeons still choose to use obsidian blades over conventional steel scalpels today and claim the finer incision means the wound heals faster and with less tissue damage hence minimal scarring. A steel scalpel has a rough cutting edge similar to a saw due to the metal's crystalline structure but this is lacking in obsidian which means the edge is completely smooth hence any incision is cleaner and more precise.
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. Black obsidian was polished and used as mirrors by the Aztecs and Greeks because of its bright vitreous lustre and it was widely traded by many ancient cultures along trade routes primarily because of its suitability for crafting into blades and tools. It's known that 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 is said to cut to the heart of the matter and can be an aid to those who are grieving or who are struggling to overcome obsessive behaviour. It 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.
Obsidian can be found in many locations around the world including several U.S states. Oregon in particular has many different types of gem grade obsidian including mahogany, red, black, rainbow and snowflake. On Mohs scale of mineral hardness it grades 5 - 5½ but being brittle means it must be handled carefully.