What a gemmologist actually does
Every post on this site ends by saying that proper identification needs a gemmologist with equipment. This one says what that equipment is and what each piece of it measures. The short version: identification works by measuring several independent physical properties and finding the one material consistent with all of them, and the reason you cannot do it at home is precision rather than cleverness.
The principle
No single property identifies a mineral. Refractive index alone leaves candidates; density alone leaves candidates; the overlap of both leaves far fewer. Identification is the intersection of measurements, each narrowing the field, until one species remains consistent with all of them.
This is why the discipline is unimpressed by a single test. It is also why home testing genuinely eliminates and does not confirm — the eliminations are real and there are never enough of them, measured precisely enough, to close the intersection.
The instruments
The loupe and the microscope. A 10× loupe first, then a binocular microscope with adjustable lighting and often with the stone immersed in a fluid to suppress surface reflections. This is where inclusions, growth structures, healed fractures, dye distribution and polishing artefacts are read. It is the most information-rich instrument in the room and the only one you own a version of.
The refractometer. Measures refractive index — how much the material bends light — against a calibrated scale, by finding the angle at which light stops passing out of a dense glass prism into the stone. Jadeite reads around 1.66, nephrite around 1.61, quartz around 1.54. Those are separated numbers and this single measurement distinguishes materials that no amount of looking will separate.
It needs a flat, well-polished area in contact with the prism. On a curved or matte object the spot method — a drop of contact liquid and a small area — may still give a usable reading, and sometimes nothing usable is available at all. Aggregates give a vague reading rather than a sharp one.
The hydrostatic balance. Measures specific gravity by weighing the object in air and then suspended in water. This is the same physics as the home displacement method and it is done on a balance reading to a fraction of a milligram, which is the entire difference. Jadeite is roughly 3.30 to 3.38, nephrite 2.90 to 3.03, serpentine 2.4 to 2.8, quartz about 2.65. Measured properly those brackets barely overlap; measured on a kitchen scale they merge into one.
The polariscope. Two polarising filters with the stone between them. It distinguishes singly refractive materials — glass, and cubic minerals — from doubly refractive ones, and shows a characteristic continuous brightness in aggregates. For separating glass from a crystalline stone this is quick and decisive.
The spectroscope. Spreads transmitted or reflected light into a spectrum and shows which wavelengths the material absorbs. Absorption features identify the colouring element and can indicate certain treatments, because colour comes from specific mechanisms with specific spectral signatures.
Ultraviolet lamps, long-wave and short-wave. Fluorescence is suggestive rather than conclusive, and it is the standard first indication of polymer in impregnated material — a chalky, patchy glow — as well as of some dyes and some coatings.
Colour filters. A dichroic filter of the kind long used in the trade can make certain chromium-related colours behave distinctively, and has historically been used as a first screen for some dyed green material. Modern dyes do not all respond, so a negative result establishes nothing.
Infrared spectroscopy. The instrument that settles the question this material most needs settled. Fourier-transform infrared spectroscopy detects organic substances — resin, polymer, wax — inside and on a stone, because organic bonds absorb in the infrared in ways minerals do not. This is how polymer impregnation is confirmed rather than suspected, and it has no home equivalent whatsoever.
Raman spectroscopy. Identifies a mineral species from the vibrational signature of its structure, non-destructively and on a very small spot. It works on aggregates and on inclusions inside a host, which makes it powerful exactly where the refractometer is weak.
X-ray fluorescence. Determines which elements are present. Useful for metals and for distinguishing materials by composition, less so for light elements.
WHAT YOU CAN TELL — with instruments, and without
· Refractive index → separates species decisively.
Needs a refractometer
· Specific gravity → separates species. Needs a
balance reading to a fraction of a milligram
· Singly or doubly refractive → glass versus
crystal. Needs a polariscope
· Resin, polymer or wax present → needs infrared
spectroscopy. No home equivalent
· Species of an aggregate → needs Raman
· Geographic origin
→ OFTEN NOT DETERMINABLE even in a laboratory,
from the object aloneWhy a polished tool is a hard case
A gemmologist’s methods were developed largely for faceted transparent gems, and a flat opaque tool is awkward for several of them at once.
No flat polished facet for the refractometer, or one too curved to seat properly.
Opacity. The spectroscope and much of the microscopy depend on transmitted light. An opaque piece of dark carbonate rock passes none.
Aggregate structure. Refractive index readings on an interlocking mass of small crystals are indistinct, because the surface presents grains in many orientations. Both jades are aggregates.
Size and shape. The object is large by gem standards and shaped for a hand, so it does not sit in the instruments conveniently.
So the tools most likely to answer are the ones least dependent on transparency and geometry: microscopy, hydrostatic weighing, and Raman. This is why laboratory identification of jade in particular leans on spectroscopy more than a textbook order of operations would suggest.
What it costs, and when it is worth it
Testing is priced per item and the price does not scale down for cheap items. For a stone tool at ordinary prices, a full laboratory identification with treatment detection will generally cost more than the object, and often considerably more.
That makes the decision straightforward in most cases and genuinely difficult in a few. A tool that is inexpensive, or that you are content to know only the bracket of, is not worth testing — the paperwork that comes in the box is not a substitute, but neither is it worth replacing at that price. A piece with real value, an inheritance, or a stone bought as Type A jadeite at a price that assumed it, is a different proposition, and there the testing is cheap relative to what it settles.
The limit, including theirs
A gemmologist can name the species, detect polymer and most dyes, distinguish glass from stone, and give you a density and a refractive index that a home measurement can only gesture at. That is a real and substantial capability, and it is the honest endpoint of every question on this site.
Two things stay out of reach even there. Natural versus synthetic is sometimes indeterminate, particularly in quartz, where the distinction rests on growth features that may be absent or ambiguous. And geographic origin generally cannot be established from the object — which means a provenance claim like the one attached to bian stone is not merely hard to check at home; it is not reliably checkable at all.
The laboratory tells you what the object is made of. It does not tell you where it has been.