# Rainbow obsidian

- Page: https://taslar.bumba.tr/en/stones/rainbow-obsidian/
- Turkish: Gökkuşağı obsidiyen
- Kind: Variety
- Classification: Mineraloids
- Crystal system: Amorphous (no crystal structure)
- Hardness (Mohs): 5–5.5
- Specific gravity: 2.35–2.6 g/cm³
- Refractive index: 1.48–1.51
- Lustre: Vitreous (glassy)
- Transparency: Translucent, Opaque
- Colours: black, iridescent bands of purple, green, blue and gold
- Streak: White
- Cleavage: None
- Fracture: Conchoidal (shell-like), with very sharp edges
- Composition: Natural glass, mostly of rhyolitic composition, very rich in silica (about 65–80% SiO₂), usually containing less than 1% water. Microscopic crystals, gas bubbles and iron-oxide inclusions affect its colour and sheen.
- Texture: Glassy, with no ordered crystal structure. May show flow banding, golden or rainbow sheen, or white cristobalite “snowflakes”.
- Value ladder: 5/9 step, Ornamental stone

A kind of obsidian that, cut the right way and held to the light, shows bands of purple, green, blue and gold on a black background.

Rainbow obsidian is a volcanic glass that at first looks like ordinary black or dark grey obsidian, but when cut and polished in the right direction shows coloured bands under a strong light. The colours do not come from dyes or pigments: they arise when microscopic rod-shaped crystals, lined up in the same direction as the lava flowed, reflect light and make it interfere. It comes mostly from the Mexican state of Jalisco and from northern California in the USA.

## How does it form?

Like all obsidian, rainbow obsidian forms when silica-rich (rhyolitic) lava cools very quickly and sets as glass. But while the lava is still flowing, very small rod-shaped crystals grow in it: according to a study published in 2001 in the Canadian Mineralogist, the journal of the Mineralogical Association of Canada, these are rods of hedenbergite (a pyroxene mineral) and plagioclase (a feldspar).

The flowing lava lines these rods up parallel to one another in the direction of flow. When the glass solidifies, the rods are frozen in place and form thin layers within the glass. Jalisco’s rainbow obsidian comes from obsidian formed in a large eruption about 95,000 years ago, while the obsidians of Glass Buttes in Oregon are about 5–5.8 million years old.

## What is it used for?

- **Ornamental stone and jewellery:** It is made into cabochons, pendants and small ornaments; because the colours only show when the stone is cut the right way, it is usually sliced with its flow lines in mind.
- **Lapidary work:** Davis Creek obsidian from California is a very popular material among lapidaries because it is easy to obtain and has beautiful colours.
- **Science:** Rainbow and sheen obsidians have been studied with microscopes and spectroscopy to understand how natural glasses turn light into colour.

## How do the colours appear?

When light strikes the aligned layers of crystals in the glass, part of it is reflected from each layer separately. These reflected light waves reinforce one another at some colours and cancel at others, so that only certain colours reach the eye. This is called thin-film interference, and it works on the same principle as the colours of a soap bubble or an oil film on water. Oregon State University’s Volcano World also attributes the blue, green, purple and bronze colours of rainbow obsidian to microscopic feldspar crystals in the glass.

## How is it cut?

According to Rock & Gem magazine, Jalisco’s rainbow obsidian is slabbed or ground parallel to the flow lines, and the best colours are seen looking perpendicular to the bands. If the cutting direction shifts slightly, or the stone is turned a little, the colours can disappear; this is why the direction of the flow lines in a rough piece is worked out before it is cut.

## How to recognise it

- In ordinary light it looks black or dark grey; turned under a strong light, bands of purple, green, blue and gold appear inside it.
- The colours lie inside the stone in layers parallel to the flow lines; they can flash and vanish when the stone is turned slightly.
- Like other obsidian, it breaks with curved, shell-like surfaces and very sharp edges.
- It is about as hard as window glass (roughly 5–5.5); a steel file can scratch it.

## What determines its value?

Value depends mostly on how vivid and how many the colours are and how even the bands are; pieces that show several colours in broad, bright bands are the most sought after. Because the colours only appear when the stone is cut in the right direction, the skill of the cutter and a good polish matter. Crack-free and larger pieces are worth more; even so, as obsidian is plentiful, it is generally an affordable ornamental stone.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

## Myths and misconceptions

- Myth: Rainbow obsidian and fire obsidian are the same stone. Fact: They are different. According to GIA, rainbow obsidian shows broad, mostly even bands of pastel colours; vividly coloured fire obsidian is rare, has so far been found at only one site in the Glass Buttes region of Oregon, and owes its colours to layers of magnetite 300–700 nanometres thick.
- Myth: Rainbow obsidian is a different stone from obsidian. Fact: Rainbow obsidian is the name given to obsidian containing fine crystal rods that create a play of colour; the stone itself is the same volcanic glass as obsidian.

## Curious questions

**Where do the colours of rainbow obsidian come from?**

According to a 2001 study by US researchers published in the Canadian Mineralogist, the colours arise when light reflected from thin layers formed by microscopic, aligned rods of hedenbergite and plagioclase interferes with itself (thin-film interference); the colours of a soap bubble form in a similar way.

## Safety

Raw or broken obsidian can have edges sharper than a razor blade and easily cause deep cuts. Wear safety glasses and gloves when breaking or knapping it.

## Did you know?

- The hedenbergite rods that make the colours are only 0.2–2 micrometres thick and 10–20 micrometres long, far thinner than a human hair.
- The obsidian that yields Jalisco’s rainbow obsidian formed in a large volcanic eruption about 95,000 years ago.
- Some lapidaries cutting Davis Creek obsidian from California slice it at an angle of a few degrees across the flow lines to catch the colours; the slab then shows them only when viewed from one direction.

## Where is it found?

- San Andrés (La Revoltosa mine), Magdalena–Tequila area, Jalisco, Mexico (Major deposit): The main source of rainbow obsidian; obsidian formed in a large eruption about 95,000 years ago
- Davis Creek, Modoc County, California, USA (Major deposit): North of Alturas; obsidian much liked by lapidaries for its coloured layers
- Glass Buttes, Lake County, Oregon, USA (Known locality): A large volcanic area yielding many kinds of obsidian, including rainbow, mahogany, gold and silver sheen

## Sources

- [Micro-analytical study of the optical properties of rainbow and sheen obsidians (Ma, Gresh, Rossman, Ulmer & Vicenzi), Canadian Mineralogist 39(1): 57–71](https://authors.library.caltech.edu/38204), Mineralogical Association of Canada, 2001
- [Obsidian](https://en.wikipedia.org/wiki/Obsidian), Wikipedia
- [Fire obsidian’s beguiling spectrum (R. Weldon & N. Renfro), Gems & Gemology 59(2), In the Spotlight](https://www.gia.edu/gems-gemology/summer-2023-in-the-spotlight), GIA, 2023
- [Mexican Obsidian of Jalisco (B. McKay)](https://www.rockngem.com/mexican-obsidian-of-jalisco/), Rock & Gem Magazine, 2026
- [Bench Tips: Cat’s Eye Obsidian (B. Rush)](https://rockngem.com/bench-tips-cats-eye-obsidian), Rock & Gem Magazine, 2019
- [Obsidian (Volcanic minerals)](https://volcano.oregonstate.edu/volcanic-minerals/obsidian), Oregon State University, Volcano World
- [Glass Buttes](https://en.wikipedia.org/wiki/Glass_Buttes), Wikipedia
- [Obsidyen](https://kureansiklopedi.com/tr/detay/obsidyen-136e9), Küre Ansiklopedi


Source: https://taslar.bumba.tr/en/stones/rainbow-obsidian/ · Licence: CC BY-SA 4.0