Mahogany Obsidian: Black Volcanic Glass Streaked With Rust Red Iron

Short answer: mahogany obsidian is natural volcanic glass in which black and red brown glass are swirled together. The brown comes from iron, present as oxide-rich bands and patches that are more oxidized than the iron in the black glass around them. It is about 5 to 5.5 on the Mohs scale, weighs in near specific gravity 2.4, and much of the supply comes from the western United States, Oregon in particular. Real mahogany is opaque and banded; a uniformly red, translucent "obsidian" is glass made in a factory.

How do you recognize mahogany obsidian?

Look for three things together. First, two colors in one piece: jet black glass and a warm rust to chocolate brown, either as flowing bands, as flame-shaped patches, or as a brown body with black veins. Geology.com describes it simply as black and brown obsidian swirled together and calls it the most common two-color combination in obsidian. Second, a glass luster on a polished face and, on any chip or broken edge, smooth curved conchoidal fracture with no grain. Third, opacity: hold a thin edge to a flashlight and the brown zones may glow a dim red at the very margin, but the body stays dark.

The bands follow flow. Viscous lava stretched and folded as it moved, and the brown streaks trace that movement, which is why they bend and pinch rather than running in straight lines. On a tower or sphere, a good piece shows those folds wrapping around the curve.

What is it made of?

Obsidian is not a mineral but glass. Geology.com explains that in very viscous, rapidly cooled lava the atoms are unable to arrange themselves into a crystal structure, leaving an amorphous material classed as a mineraloid; the mineraloids guide covers the category. The chemistry is that of rhyolite: Geology.com's rhyolite article notes that granitic magma erupting effusively produces rhyolite, or obsidian if the lava cools rapidly, the two having similar compositions but different cooling histories.

The silica, gravity and index figures come from the International Gem Society's obsidian data, which also lists mahogany as a named variety.

Why is mahogany obsidian brown instead of black?

The short version is iron. The International Gem Society attributes mahogany's reddish brown color to iron impurities, and the German gem lab EPI describes the variety as black obsidian with brown, iron oxide-rich banding in its comparison of natural obsidian and glass. In the same list EPI explains ordinary black obsidian as black because of ore inclusions.

The useful way to picture it comes from basic iron oxide chemistry. The Quartz Page's inclusions chapter notes that magnetite, the black iron oxide, forms and is stable only where oxygen activity is relatively low, and otherwise tends to oxidize to hematite, which forms under oxidizing conditions and is black or red in fine aggregates. Hematite is iron oxide, Fe2O3, and in powder it is always reddish, which is why Geology.com's hematite article treats its reddish streak as the key identification clue. Brown glass streaks are where the dispersed iron ended up in that more oxidized, rust colored form; the black glass kept it reduced. The hematite guide covers the mineral itself. Treat this as the general mechanism rather than a measured analysis of any one piece: the exact particles vary from flow to flow.

Where does mahogany obsidian come from?

Obsidian occurs wherever silica-rich lava cooled quickly, but the gem and lapidary trade leans heavily on North America. Geology.com states that most obsidian used in the jewelry trade is produced in the United States, and that in the US it occurs only west of the Mississippi because there has been no geologically recent volcanism in the east. The International Gem Society names Oregon as a source of fire, mahogany and rainbow obsidian.

One Oregon site shows how mixed a single field can be. Geology.com illustrates a set of specimens from the Glass Butte rockhounding site in central Oregon that includes double flow, rainbow, black, pumpkin, mahogany and gold sheen obsidian, all from a small area. Northern California's Modoc National Forest is another obsidian district, and collecting there is regulated: the forest's geology page states that mining obsidian for personal use requires a free-use authorization permit. Labels on finished mahogany pieces seldom name a flow, so "USA" or "Mexico" is the most you should expect.

What are the fakes, and how do you spot them?

Mahogany itself is cheap enough that it is rarely imitated directly. The trouble is its neighbor in the shop tray: "red obsidian". EPI is blunt that red obsidian is completely unknown in nature and that glasses of that color are always artificial. Their description of the product is a good checklist: sold mostly as beads and tumbles, natural looking to the naked eye, but under a magnifier full of round gas bubbles and red inclusions.

How to keep the two apart:

1. Color pattern. Mahogany is black plus brown in flowing bands. Solid cherry red or orange throughout is not mahogany and, per EPI, not natural obsidian at all.
2. Transparency. Real mahogany is opaque in normal light. Translucent red or bright, saturated colors point to manufactured glass.
3. Bubbles. Natural obsidian can contain bubbles too; the International Gem Society lists elongated, torpedo-shaped bubbles, often in parallel arrangement, among its inclusions. Round, randomly scattered bubbles in a red stone are the factory tell.
4. Price and story. EPI also documents glassy industrial slag sold as "blue obsidian" and melted glass sold with Mount St. Helens stories. A dramatic origin story on a strangely colored "obsidian" is a reason to walk away.

Natural look-alikes are a different matter. The International Gem Society notes that jet, chalcedony and black tourmaline can be confused with obsidian, and that refractive index separates them because the gravity ranges overlap. For the black-and-white banded confusion, see obsidian vs onyx.

Is it durable enough to wear?

Only gently. Geology.com puts obsidian's hardness at about 5.5, says it lacks toughness and chips on impact, and calls it an inappropriate stone for rings and bracelets; pendants, earrings and brooches are the safer settings. The typical damage is predictable: bracelet beads with chipped edges and tumbles that knocked against harder stones in a bag.

How do you care for mahogany obsidian?

The International Gem Society points out that household dust has a hardness of 7, harder than obsidian, so wiping a dusty sphere with a dry rag slowly scratches it; use a lint-free cloth and mild soapy water instead. It also warns that obsidian may contain inclusions that could expand when heated and advises against mechanical cleaning systems. Store pieces separately from quartz and other harder stones, and keep towers where they cannot topple, since a fall onto tile usually ends with a conchoidal chip off the point.

What is mahogany obsidian associated with in tradition?

In crystal traditions mahogany obsidian is treated as a gentler, warmer version of black obsidian, associated with grounding and steadiness, and often linked with the root and sacral chakras. These are beliefs attached to the stone, not properties of the glass.

What to look for when you buy mahogany obsidian

Pick for pattern contrast and finish. The best pieces have clear separation between black and brown, with bands that flow across the shape instead of a muddy brown overall. On a polished face, hold the piece at a low angle to a window: a good polish shows an unbroken reflection without pits or orange-peel waviness. Check points and edges of towers for chips. And remember the gravity: a genuine piece of this size feels noticeably lighter than quartz of the same volume.

• Mahogany Obsidian Crystal Tower Volcanic Glass Obelisk: 121 mm tall and 32 mm wide, a size where the flow banding wraps around four faces; check the tip in the photos for chips.
• Mahogany Obsidian: small, inexpensive pieces that are the easiest way to see the black and brown swirl up close and test the luster yourself.
• Black Obsidian Crystal Sphere Polished Volcanic Glass: plain black glass from the same family, useful beside a mahogany piece to see exactly what the iron banding adds.

For shapes, the towers and obelisks guide explains what separates a well cut point from a hasty one, and the main obsidian guide covers the other varieties.

Frequently asked questions

Is mahogany obsidian natural?

Yes. It is volcanic glass that cooled too fast to crystallize, with iron oxide-rich bands that turned red brown. Uniformly red or translucent "obsidian" is a different matter: natural red obsidian is unknown, and those products are manufactured glass.

What makes mahogany obsidian brown?

Iron. The brown bands carry iron in a more oxidized, rust colored state, close to hematite in character, while the surrounding black glass holds iron in a reduced, dark form. The two swirled together as the thick lava flowed.

Where is mahogany obsidian found?

Mostly in the western United States, with Oregon a named source, and in other young volcanic regions such as Mexico. Glass Butte in central Oregon yields mahogany alongside rainbow, gold sheen and black obsidian in a small area.

How hard is mahogany obsidian?

About 5 to 5.5 on the Mohs scale, softer than quartz and steel. It also lacks toughness, so it chips when knocked. It suits pendants and display pieces better than rings or bracelets.

How do you tell mahogany obsidian from red glass?

Check pattern and transparency. Mahogany is opaque black with flowing brown bands. Factory red glass is evenly colored, often translucent, and under a loupe shows round, scattered gas bubbles. Natural obsidian bubbles tend to be elongated and aligned.

Sources

• Geology.com, Obsidian: https://geology.com/rocks/obsidian.shtml
• Geology.com, Rhyolite: https://geology.com/rocks/rhyolite.shtml
• Geology.com, Hematite: https://geology.com/minerals/hematite.shtml
• International Gem Society, Obsidian value, price and jewelry information: https://www.gemsociety.org/article/obsidian-jewelry-and-gemstone-information/
• EPI Institute for Gemstone Testing, Obsidian: natural product versus artificial glass: https://epigem.de/en-us/informations/imitations/fake-glass-imitation-obsidian.html
• The Quartz Page, Inclusions: http://www.quartzpage.de/inc_text.html
• USDA Forest Service, Modoc National Forest geology: https://www.fs.usda.gov/r05/modoc/natural-resources/geology