Gemstone or glass: the indicators a photograph can show

Moulded glass and cut mineral are separated in a laboratory by one measurement that takes a minute. Without instruments you are left with indicators: features that lean one way, carry different weights, and settle nothing individually. A photograph can show most of them, which is why every reading from the tool reports them as indicators present, indicators absent and indicators not readable, rather than as a verdict.

Below are the six that a picture can carry, heaviest first.

Rounded voids in the body

Weight: heavy. Glass is poured and can trap gas, and trapped gas is spherical or stretched into a smooth teardrop. A crystalline host does not produce spheres. Several round voids in one piece is the strongest single indicator available without instruments.

What it cannot settle: small voids need magnification, so their absence in a phone photograph is close to meaningless. Backlighting exaggerates them, and a dust speck on the surface can read as a void in the body. A magnified look through a 10x loupe is the check worth doing.

Curved swirl or flow lines

Weight: heavy. Smooth curved streaks running through the interior are flow lines from a liquid that set while moving. Colour inside a crystalline host varies in straight zones and parallel bands instead, along the directions in which it grew. Curved versus straight is the distinction to hold onto, and it is visible in a photograph if light passes through the piece.

What it cannot settle: a strongly figured background seen through a transparent stone can imitate swirls. Shoot against a plain neutral card before drawing anything from this one.

Angular internal features, as the counter-indicator

Weight: heavy, in the other direction. Straight needles with flat reflective faces, feathery veils, and features arranged along distinct directions are grown into a crystalline host. Moulded glass does not make them. When a photograph shows angular directional inclusions, that argues against a moulded piece more firmly than most indicators argue for one.

What it cannot settle: which mineral the needles are, and therefore which family the host belongs to. That still needs magnification and, past a certain point, instruments.

Rounded facet junctions

Weight: moderate. Where two faces meet on a crisply cut stone, the junction reads as a line. On a moulded piece it often reads as a narrow rounded band instead, because the mould left it soft.

What it cannot settle: wear does the same thing. A cut mineral carried in a pocket or worn daily for decades has softened junctions too, and no photograph separates a soft junction from a worn one. This indicator is only useful next to the others.

Mould seams

Weight: moderate when present, nil when absent. A moulded piece can carry a faint straight line running across a face or around the girdle, where the two halves of the mould met. A seam that survives polishing is unusual, so its absence proves nothing at all.

What it cannot settle: a surface scratch looks the same in a photograph. A seam runs continuously and follows a plane; a scratch wanders and stops.

Luster and wear pattern

Weight: light. Glass often reflects slightly softer than the mineral families it imitates, and it abrades more readily, so an older piece can show a scuffed table and dulled edges while its setting is still crisp. Both of these are comparative judgements, and a photograph gives you nothing to compare against.

What it cannot settle: nearly anything, alone. Treat luster as the tie-breaker between two readings, never as the reading.

What to do with a set of indicators

Read them together and notice which way the heavy ones point. Rounded voids plus curved swirls plus soft junctions is a consistent set, and a reading that names “glass or moulded imitation” at the top of its list on that basis is reasoning properly. One soft junction on its own is not evidence of anything.

Then notice what the frame could not show. On a stone held in a setting this check is nearly empty, because the features that give a moulded piece away sit at the girdle and in the body, and the setting covers both. A profile photograph of the same piece, or the stone out of its setting, changes that more than any amount of re-reading the first frame.

Where this stops, and what to do instead

No combination of these six is a measurement. What settles the question is a refractive index reading, taken in person, by someone qualified, in less time than reading this took.

Safe things to do before that appointment: photograph the piece again in daylight against a plain card, once face up in reflected light and once with the light behind it; look through a 10x loupe at whether the voids are truly spherical and whether the junctions are soft all the way round; put it on a jeweller’s scale if the stone is loose and its size is known. Nothing that scrapes, heats or soaks the piece belongs on that list, and none of it is needed.

Upload your photograph to get the indicators laid out for your own piece, or read how to identify a gemstone from a picture for the order the rest of the clues come in.