Gold Sheen vs Black Obsidian: Sheen Cause, Look & Choosing

If you pick up two obsidian beads in a shop and one of them flares gold under the spotlight while the other stays a flat matte black, what you are seeing is not a different stone. It is the same volcanic glass, formed in the same way, in the same kind of lava — except one of them was lucky enough to catch a sheet of iron-oxide platelets as it froze.

This piece is a quick mineralogical pass through both. Where the glass comes from, what makes the gold sheen actually shimmer, and how to choose between the two when both options are on the table. Same care, same hardness, very different visual presence.

What obsidian actually is

Obsidian is not, strictly, a mineral. A mineral has a defined chemical formula and a repeating crystal lattice. Obsidian has the first — it is dominantly SiO₂, around 70–75%, with smaller amounts of Al₂O₃, Na₂O, K₂O and trace iron — but not the second. It is a mineraloid: a naturally occurring glass that has the chemistry of rhyolite but never had time to crystallise.

The reason is cooling speed. When felsic (silica-rich) lava reaches the surface, it is viscous and full of dissolved water. If it cools slowly, that silica organises into quartz, feldspar and other crystals; you get granite or rhyolite. If it cools fast enough — in a thick flow that hits cold rock or water, in days rather than years — the silica freezes mid-shuffle into a disordered glass. That glass is obsidian. The same chemistry, two completely different textures, decided by the cooling rate.

Because there are no crystal grain boundaries, obsidian breaks with what mineralogists call a conchoidal fracture: smooth, curved surfaces with razor-thin edges. The edges are sharp enough that Mesoamerican toolmakers used flaked obsidian as surgical blades, and modern surgeons have demonstrated obsidian scalpels that cut finer than steel. This matters for jewellery only as a warning: a chipped obsidian bead can have a genuinely sharp edge.

Why black obsidian is black

Pure silica glass would be colourless. Obsidian is dark because it carries trace iron — typically less than 2% by weight — in two forms: dissolved Fe³⁺ ions in the glass network, and microscopic crystallites ("nanolites") of magnetite and hematite that nucleated as the glass cooled. The dissolved iron absorbs across the visible spectrum, producing the dark grey-to-black ground colour. Held up to a strong light, a thin chip of obsidian reveals itself as deep brown-grey rather than truly black.

When the iron-rich nanolites stay randomly distributed, you get plain black obsidian. When they line up in sheets during the lava's flow, you get the gold sheen variety.

Why gold sheen obsidian shimmers

Gold sheen is the same glass, with one extra geological event in its history: shear. As the cooling lava was still able to flow, dissolved iron precipitated out as thin platelets — mostly magnetite (Fe₃O₄) and hematite (Fe₂O₃) — and the ongoing motion of the flow aligned those platelets along sheets parallel to the direction of movement.

The result is an internal sandwich of mineral platelets, all facing roughly the same way, all close enough together to act as a partial reflector. When white light hits the polished surface of a gold sheen bead, the platelets reflect a coherent gold-bronze flash back to the eye. Tilt the bead and the flash moves. That moving metallic glint is called schiller in mineralogy, the same word used for moonstone adularescence and labradorite labradorescence, although the mechanism here is straightforward platelet reflection rather than thin-film interference.

One practical consequence: gold sheen only "works" from certain angles. Each bead has its platelet layers oriented in one direction, so beads on a strand will catch the light unevenly — some flash bright while neighbours stay dark. This is correct, not a defect, and good cutters will try to orient drill holes so that the sheen sits across the wrist rather than along it.

Where each one comes from

Both varieties form anywhere felsic volcanism has produced thick, slow flows of rhyolitic lava. Plain black obsidian is found across Mexico, the western United States (Oregon, California, Idaho), Iceland, Armenia, Turkey, Japan and New Zealand. The major source of gold sheen obsidian commercially used today is the Mendoza province of Argentina, with additional historical deposits in the Sierra de Pachúca, Hidalgo, Mexico (the source most Mesoamerican gold sheen artefacts trace back to), and a small Oregon source near Glass Buttes.

How to choose between them

The two read as completely different objects on the wrist even at the same bead size. Black obsidian is matte presence: it absorbs light, sits dark against skin, photographs as a clean silhouette. Gold sheen is animated: it changes minute to minute as you move, switching between black and bronze depending on how the wrist faces a light source. Neither is fussy on a colour palette — both work with any clothing — but they ask for different things from the rest of an outfit.

• If your wardrobe is graphic and quiet. Black obsidian reads as a graphic full stop, a strong dark line against skin. It pairs well with monochrome, with stark metal hardware, with anything where you want the stone to disappear except as form.
• If your wardrobe carries warm tones. Gold sheen reads as a metallic accent. The bronze flash sits comfortably alongside gold hardware, camel, oxblood, anything in the warm spectrum.
• If you want the strand to do something on its own. Gold sheen has a built-in performance: it catches every direct light source in a room. Black obsidian holds still. The first is louder, the second more disciplined.
• If you stack. Black obsidian functions as a base — it absorbs visual noise and lets other stones speak. Gold sheen wants room; it competes with anything reflective alongside it.

Care notes that apply to both

Obsidian is softer than quartz — Mohs 5 to 5.5 — which means it scratches against most household stone surfaces, against unset diamonds in stacked rings, and against any quartz or topaz it shares space with in a drawer. Store strands separately, ideally rolled in a soft cloth pouch. Avoid impact: obsidian's conchoidal fracture means a hard knock against a tile floor can produce a chip with a working-knife edge.

Cleaning is simple: lukewarm water, a drop of pH-neutral soap, a soft brush. No ultrasonic cleaners (vibration finds invisible flow-line fractures in the glass). No steam. No solvents. Both varieties tolerate skin oils, perfume and sunscreen without issue, but wipe the strand with a dry cloth after wear to keep the polish even.

How BE. grades each

The Crystal 4T framework — Tone, Transparency, Texture, Trace — lands differently across the two. For black obsidian, Tone is graded against the depth of the matte, with extra credit for stones that hold a dry, true black under direct light rather than washing out to grey. Texture is graded against polish evenness and the absence of micro-fractures left from cutting. For gold sheen, the dominant axis shifts to what BE. calls Trace: the brightness, coverage and directional cleanness of the sheen layer. A good gold sheen bead reflects across most of its surface, not just one face. Each strand ships with a Stone Origin Card noting source region and the geological feature — plain glass or platelet-aligned glass — that defines it.

References

• Mindat — Obsidian data
• Wikipedia — Obsidian (composition, sources)
• GIA Gem Encyclopedia — Obsidian varieties
• USGS — Volcanic glass & rhyolite
• Webster, R. (2002). Gems: Their Sources, Descriptions and Identification, 5th ed. Butterworth-Heinemann.
• Schumann, W. (2009). Gemstones of the World, 4th ed. Sterling.