Obsidian

Start with the surface and the break. A genuine piece of obsidian looks and feels like glass: it has a bright vitreous luster, smooth curved (conchoidal) fracture surfaces, and edges that can be razor-sharp. Held up to a strong light, a thin edge of black obsidian often glows a deep translucent brown rather than staying solid black. It is moderately hard — it will scratch ordinary glass — and gives a white streak on an unglazed tile.

The biggest clues are what obsidian does NOT show. There are no visible crystals, no grains, and no banding of the sort you see in agate or onyx, because it never crystallized. Its weight feels ordinary for a rock (it is relatively low in density, close to common glass) and it is not magnetic. If you see crystal faces, a sugary or grainy texture, sharp color bands, or feel real heft, you are probably looking at something else.

Most obsidian is jet black, colored by tiny amounts of iron and magnesium dispersed through the glass. But several distinctive varieties exist. Snowflake obsidian is black glass dotted with pale gray-white 'snowflakes' — these are radiating clusters (spherulites) of the silica mineral cristobalite that began to crystallize within the glass. Mahogany obsidian shows swirling reddish-brown and black patterns, the brown coming from iron oxidation. Rainbow, gold-sheen and silver-sheen obsidian display a shimmering iridescence: this is not surface coloring but light scattering off countless microscopic mineral or gas-bubble inclusions aligned in flow layers within the glass.

Other named types include snowflake-free gray and green obsidian, and 'mahogany' grading into nearly solid black. Apache tears are small, rounded nodules of translucent obsidian weathered out of grayish volcanic rock. All of these are the same basic material — volcanic glass — distinguished only by inclusions, oxidation and the way light interacts with them.

Obsidian forms when felsic lava — lava rich in silica, the same chemistry that elsewhere makes granite and rhyolite — erupts at the surface and chills almost instantly, for example along the cooled margins of a thick lava flow or where lava meets air or water. Crystals need time and atomic mobility to grow; when cooling is fast enough, the silica-rich melt freezes into a solid before any orderly lattice can develop, locking the atoms into the disordered, glassy state.

Because it is a glass rather than a crystalline solid, obsidian is geologically short-lived. Over thousands to millions of years it slowly devitrifies — the glass gradually crystallizes — and absorbs water, turning cloudy and eventually breaking down. That is why obsidian is found around relatively young volcanic centers and is essentially absent from very old rock formations. The same silica-rich magma that quenches into obsidian will, if it cools slowly underground, instead form coarse crystalline rock; obsidian is simply the fast-cooled, glassy outcome.

Obsidian has a long cultural history as a protective stone. In many crystal and folk traditions it is described as grounding and as a 'psychic shield' that absorbs negativity, and black obsidian in particular is associated with protection and with facing difficult truths. Polished obsidian's mirror-like surface gave it a role in scrying and divination across several ancient cultures, and it has been carved into mirrors, masks and amulets for millennia.

It is worth being clear that these metaphysical associations are cultural and spiritual rather than scientifically established. Obsidian has no proven medical or psychological effect, and it should never be used in place of professional medical or mental-health care. Enjoyed for its beauty, its history and its symbolism, it is a striking stone — but its 'healing' properties are a matter of belief, not of demonstrated fact.

The most common thing mistaken for obsidian — sometimes innocently, sometimes deliberately — is ordinary manufactured glass, including black bottle glass and 'slag' glass. Both are glass and both break conchoidally, so fracture alone will not separate them. The tells are in the imperfections. Natural obsidian usually contains subtle, irregular flow lines and tiny stretched or teardrop-shaped bubbles and inclusions, and its color is rarely perfectly even. Manufactured glass tends to look too perfect: uniform, saturated color (often a slightly bottle-green or unnaturally pure black when backlit), and round, evenly distributed air bubbles rather than stretched ones.

Backlighting helps: many natural black obsidians glow warm brown at the edges, while a lot of imitation glass glows green or stays flatly black. Mold seams, a suspiciously regular shape, or a perfectly smooth molded (rather than fractured) surface point to a manufactured piece. None of these tests is foolproof on its own, so weigh several clues together, and treat extremely cheap, flawless 'obsidian' beads with healthy skepticism.

Obsidian is easy to clean — warm water and a soft cloth are all it needs — but it must be handled gently. Although it is hard enough to scratch glass, it is brittle and prone to chipping, and freshly broken edges can be genuinely sharp enough to cut skin, so handle raw flakes with care and keep them away from children. Store obsidian separately from harder stones such as quartz and topaz, which can scratch its glassy surface.

Avoid hard knocks, and avoid sudden temperature changes, which can stress the glass. Because it is a glass that slowly absorbs water and devitrifies over geologic time, no special action is needed on a human timescale, but prolonged harsh chemical exposure is best avoided to keep the polish bright.