Mahogany obsidian — swirl of dark chocolate lava
Mahogany obsidian – obsidian with character: black volcanic glass tinted with rich burnt mahogany bands and spots. Warm brown tones arise from fine iron oxide particles and iron-enriched layers arranged along the lava flow. Turn the stone in the light – the swirls seem to move like smoke under glass. If regular obsidian is midnight with a tuxedo, mahogany obsidian is midnight with a leather jacket.
Identity and name 🔎
What it is
Mahogany obsidian is a variety of obsidian — natural volcanic glass — recognized by its warm brown color combined with black. Since obsidian is amorphous (lacking a long-range ordered crystal lattice), it is classified as a mineraloid, not a mineral.
Why "mahogany"?
The color resembles polished mahogany: richly reddish brown with subtle variations. Geologically, this palette is created by iron oxides dispersed under the glass and minor oxidation changes during flow and cooling.
How the pattern forms 🌊
Flow banding
As silica-rich lava flows, microcrystals (microlites), bubbles, and iron-bearing melt segregate into bands. When the lava solidifies into glass, these bands "freeze" as visible mahogany and black stripes.
The role of iron
Fine particles of iron oxides (hematite, magnetite, or their alteration products) and iron-enriched glass layers give brown tones. The particle size and concentration determine how deep and "wood-like" the color appears.
Sometimes — and "additionally"
Over time and with heat exposure, part of the glass can crystallize into radial spherulites (like "snowflakes" in other obsidians). They occur less often in mahogany varieties but can appear as subtle grayish rings.
Recipe: molten silica + iron swirl + rapid cooling → chocolate on black glass.
Colors, patterns, and varieties 🎨
Palette
- Black — the base glass.
- Mahogany brown — from rust to "brick" redness.
- Umbra — darker, iron-rich bands.
Thin edges in strong light are usually translucent "tea brown" — a classic obsidian feature.
Pattern glossary
- Swirls and "feathers": fluid brown strokes on a black background.
- Stripes: parallel to the flow, following the lava movement.
- Spots: round color "pools" where the iron melt briefly paused.
Photo tip: side light at ~30° and a white reflective card on the opposite side — brown warms up, and banding becomes clearer without glare.
Physical and optical properties 🧪
| Property | Typical value / note |
|---|---|
| Type | Natural volcanic glass (from rhyolitic to dacitic composition) |
| Structure | Amorphous (non-crystalline) |
| Hardness | ~5–5.5 (medium hard but brittle) |
| Relative density | ~2.30–2.45 |
| Fracture | Shell-like — curved "shells"; edges can be razor-sharp |
| Refractive index | ~1.48–1.51 (variable) |
| Luster | Glassy; resinous in air-exposed crusts |
| Transparency | From opaque to transparent in thin edges |
| Magnetism | Not magnetic as a whole; brown zones may contain minor iron oxides |
Under the magnifier 🔬
Flow streaks
At ~10× magnification, look for fine parallel streaks and stripes in both the black and brown areas. These are rows of microlites and strings of tiny bubbles "caught" in the flow.
Iron hints
Brown spots often have a soft, cloudy texture because light is scattered by submicroscopic iron phases. You won't see separate crystals — only a subtle haze compared to the glassy black zone.
Spherulites (occasionally)
May have small gray "snowflake" rosettes — fine cristobalite radial needles. Rarer in mahogany obsidian than in "snow" obsidian.
Similar stones and how to tell them apart 🕵️
Mahogany jasper / agate
Microcrystalline quartz (hardness ~7). Feels harder, polishes differently, and lacks glassy conchoidal waves when broken like obsidian.
Basalt (brown "dusty")
Fine crystalline rock; from matte to semi-glassy; edges are not "glassy." Crystals visible under a loupe, not solid glass.
Industrial glass / slag
Can mimic color but often has porous bubbles, metallic streaks, or swirling metallic sheen. Environmental context (near furnaces) helps.
Dyed composites
Uniform "neon" browns or repeating patterns indicate artificial origin. Natural mahogany obsidian patterns are organic and non-repeating.
Quick check
- Glassy luster + tea-brown edges in strong light.
- Curved conchoidal fracture waves and sharp flakes.
- Flow-patterned brown swirls, not granular or "agate"-like bands.
Home tests
Illuminate a thin edge with a flashlight: mahogany zones glow warm brown, black looks smoky. Under a loupe, you will see flow streaks, not "sugary" grains.
Localities and context 📍
Where found
Found around felsic volcanic centers. Known sources: Western US states (e.g., Oregon obsidian fields), Mexico, and other rhyolitic provinces worldwide. Forms where lava cools quickly — in domes and edges.
Geological "neighbors"
Travels with other obsidian varieties (black, "sheen", "rainbow"), rhyolite, perlite (hydrated obsidian), and pumice formed from the same eruption. Trace element "fingerprints" can link pieces to their volcanic sources — a true archaeological superpower.
Care and storage 🧼
Everyday use
- Obsidian is brittle; avoid drops and strong impacts.
- Edges can be razor-sharp. Handle green fracture pieces like a knife.
Cleaning
- Lukewarm water + gentle foam + soft cloth; rinse and dry.
- Avoid sudden temperature changes — glass dislikes "thermal whacks."
Exposure and storage
- Keep separate from harder quartz/corundum to preserve the polished surface.
- Side light at about 30° beautifully "pulls out" the brown color.
Practical tests 🔍
"Tea brown" showcase
Hold a thin edge against a spotlight: "black" turns into warm brown, especially in mahogany zones. Never gets boring.
Searching for shell-like ripples
Raking light will reveal curved ripples around a small fracture. Each wave is a frozen shock wave of fracture.
A little joke: mahogany obsidian proves that lava can do latte art.
Questions ❓
Is mahogany obsidian a separate mineral?
No — it is a variety of obsidian (natural volcanic glass), whose color and pattern are created by iron-rich components and flow textures.
Can it show "sheen" or rainbow effects?
Sometimes, if there are thin layers of bubbles or nanolayers, but classic mahogany obsidian is valued for color contrast, not iridescence.
How to distinguish from dyed glass?
Look for organic, non-repeating flow patterns, tea-brown edge translucency, and sides without porous bubbles. Dyed composites are usually uniformly colored with repeating motifs.
Is it suitable for tools or jewelry?
Historically, obsidian provides excellent blades. In jewelry, it polishes to a high shine and looks impressive — just remember its brittleness, so protective settings are wise.
Does the color fade?
No — ore is characteristic of glass and iron phases. Keep the surface polished, and the contrast will remain sharp.