Rainbow Obsidian

Rainbow obsidian, also known as fire obsidian, is a type of natural glass that gets its name from the iridescent sheen that appears on its surface when it is cut and polished. This iridescence is caused by the presence of tiny inclusions of magnetite, which refract light in different directions.

Obsidian is a naturally occurring volcanic glass. It is formed during the eruption of felsic lavas, which are distinguished by having high concentrations of the chemical element silica. Because of their high silica content, felsic lavas do not behave like the mafic, or silica-poor, lavas we see on the island of Hawaii.

Silica forms bonds with oxygen in lava creating linked molecule chains. These linked molecule chains are called polymers and the process of forming them is called polymerization. Polymers increase the viscosity, or resistance to flow, of the lava.

Polished Rainbow Obsidian.
 Polished Rainbow Obsidian.  Photo: La Roche Mère

Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color.  Obsidian with multicolored iridescence caused by inclusions of magnetite nanoparticles. Obsidian is often formed in rhyolite lava flows where the lava cools so fast that crystals do not have time to grow.


Glass, unlike crystals, has no regular structure and therefore fractures in smooth, curved shapes. The intersections of these fractures can form edges sharper than the finest steel blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades. Even today, surgeons use the knife sharp edges in operations. 

What Causes the Color of Rainbow Obsidian?

The color of rainbow obsidian is caused by the presence of tiny inclusions of magnetite, which refract light in different directions. These inclusions are typically less than 0.01 millimeter in size and are arranged in a regular pattern. When light hits the inclusions, it is scattered in all directions, creating the iridescent sheen that is characteristic of rainbow obsidian.

The color of the iridescence depends on the size, shape, and spacing of the inclusions. For example, inclusions that are arranged in a regular pattern will produce a more uniform iridescence, while inclusions that are more randomly arranged will produce a more chaotic iridescence. The color of the inclusions can also affect the overall color of the rainbow obsidian. For example, magnetite inclusions that are black will produce a black iridescence, while magnetite inclusions that are red will produce a red iridescence.

The iridescence of rainbow obsidian is a result of a phenomenon called thin-film interference. Thin-film interference is a phenomenon that occurs when light passes through a thin layer of material. The light waves are reflected off of the top and bottom surfaces of the layer, and the waves interfere with each other. This interference can produce a variety of colors, depending on the thickness of the layer and the wavelength of the light.

In the case of rainbow obsidian, the thin layers are created by the magnetite inclusions. The magnetite inclusions are less dense than the surrounding glass, so they cause the light to bend as it passes through them. This bending creates the thin layers that are responsible for the iridescence.

California Rainbow obsidian

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