Obsidian is a glassy black substance formed when lava cools in a short amount of time. Due to the rapid rate of cooling, very few crystals can form in the material. This makes obsidian essentially a natural glass, and it has proven useful because of its lack of crystals. In particular, it can be cut to have very sharp edges. It was a favored material for weapons during the Stone Age, and is still used in some surgical instruments today.
It’s not hard to recognize obsidian. It is often black in color, and even in unpolished form it is often quite shiny. It’s made up of about 70% silicon dioxide or more, which accounts for its color. However, if obsidian mixes with some minerals it can take on other colors. For example, a high level of iron or magnesium may cause the rock to appear dark green instead of black. Some has a snowflake pattern that results from cristobalite forming on the top.
In other instances, the substance bubbles and forms into layers. This can create rainbow or golden obsidian. These two variants are more difficult to find in great quantity.
Obsidian is not really a mineral or a rock, because it does not contain crystalline structures. Some refer to it as a mineraloid. A mineraloid is a mineral-like substance that lacks crystal structures. Other examples of mineraloids include opals and pearls.
It is a relatively soft substance, with only a rating of about 5 to 5.5 on the Mohs Scale. However, despite its softness, it was a preferred material for manufacturing weapons since it could be refined to have a very sharp edge. One can still find arrowheads throughout North America made from obsidian. It could also be used for decorative purposes. The stone statues on Easter Island are made from this material.
Some surgeons, particularly cardiothoracic surgeon, use scalpels and surgical knives made of obsidian. The sharper the knife can be, the less damage it does to the tissues when cutting. These special surgical cutting instruments often lessen healing time because of their sharpness.
One can find obsidian in numerous places on the earth, particularly in areas like quarries, or where there has been volcanic activity. However, it does deteriorate into tiny glass crystals over time. There are no examples from earlier than the Cretaceous Period, 145 to 400 million years ago.