Polished Rock Identification for Tumbled and Cut Stones

Hold the stone under a lamp and tilt it. On a rough rock you would read luster from the broken face; here every material reads glassy, so ignore the shine and look through and into the stone instead. The table pairs each rough-rock clue with its polished replacement.

Banding is the strongest survivor. Agate shows fine concentric or parallel bands that follow the shape of a former cavity, often with a cloudy center, and the bands stay visible against a lamp. Onyx is the same with straight parallel bands. Jasper has no bands, is opaque, and keeps small pits and dull patches because its grain is finer and less even; a jasper and an agate from the same batch are told apart by whether light passes through the edge.

Dendrites are the black fern-like growths in dendritic agate; they sit at one depth and look flat when you tilt the stone, unlike moss agate's green inclusions, which float at different depths. Rutilated quartz shows straight golden needles; phantom quartz shows ghost outlines of an earlier crystal.

Grain boundaries survive in anything made of fused grains. Tumbled quartzite looks like a sugar cube under a thin glaze, and green aventurine adds flashing mica flakes. Nephrite shows no grains but a fine fibrous sheen when you roll it under the lamp. Tiger's eye shows a single bright band that slides across the stone as you tilt it, called chatoyancy; labradorite shows a sheet of blue, green or gold flash, called schiller, that appears from certain angles and vanishes from others.

Translucency pattern is the last check. Hold the stone against a phone torch. Agate glows with its bands, chalcedony glows evenly, rose quartz glows soft pink and cloudy, quartzite glows at the grain edges, jasper and most howlite stay dark, and glass glows evenly with no internal structure at all.

Dye concentrates where the stone was porous. Dyed howlite shows bright blue or green with the gray spider-web veins darkest of all; natural howlite is white with gray veins. Dyed agate shows band colors that do not occur together in nature, with the dye deepest in the cloudiest band and in any crack. Diffuse light shows the concentration; flash hides it.

Glass gives itself away at the edge and inside. Look for round bubbles, curved swirl lines, and chips on the rim that break in clean conchoidal flakes. Goldstone is glass with copper flakes, opalite is glass with a milky blue glow and an orange flash in transmitted light, and sea-glass pebbles tumble to the same frosted finish as quartz. None is a rock, and the identifier will say so when the bubbles are in the photo.

A photo cannot show hardness, density, authenticity or value. It cannot confirm jade, tell natural from heated color, or prove a stone is untreated. For a suspected gem, never scratch or heat it; photograph it, ask the follow-up question about the specific clue, then see a jeweler if the answer matters.

Polished surfaces reflect, so the usual outdoor shot returns a white glare and a mirror image of your phone. Use diffuse light from a window with paper taped over it, or a lamp bounced off a wall, and a dark matte background so the edge is visible. Skip the wet shot; the surface is already as wet-looking as it will ever be.

The option tells the model that the uniform shine and deepened color come from processing, so it stops treating luster as evidence and weighs pattern, translucency and inclusions instead. Each candidate comes back with its visible clue, such as concentric banding, sugary grain mosaic or spherical bubbles. If two candidates share the same clue, the deciding photo is almost always the back-lit one or the edge close-up, and the follow-up question is where to ask about the clue it did not mention.

A tumbled stone also has no provenance. Even if it came from a Lake Superior beach bag, the polish removed the rind, so read the candidates as material names and use the location pages for the beach itself.