A kitchen-scale gem testing kit narrows more than a refractometer

Last updated: August 2026

Every gemology course starts you at a refractometer. We ran all 132 species in our database through an identification engine to check whether that is actually the right first purchase. It isn't — and the reason is more useful than the headline.

candidates left by a scale, a magnet
and a UV torch

candidates left by a refractometer used alone

candidates left when you own both — and 26 of 132 identified outright

What each kit actually narrows

Each row simulates measuring every one of the 132 species with that kit, then counts how many candidates survive. Median is the typical case — half of stones do better, half worse. Solved outright means exactly one species remained.

Kit	Median left	Solved outright	Narrowed to ≤5	Tools
Colour alone	132	0	0	Your eyes
Polariscope	75	0	0	Two polarising filters
Specific gravity	24	4	10	Kitchen scale + water
SG + magnetism	20	7	28	+ rare-earth magnet
SG + magnetism + UV	8	10	46	+ UV torch
Refractometer alone	22	0	4	Refractometer + RI liquid
Refractometer + SG	13	9	29	Refractometer + scale
The cheap kit + a refractometer	4	26	75	All of the above

Two things stand out. A refractometer alone leaves 22 candidates, while that simple equipment leaves 8. But add the refractometer to that cheap kit and you land at 4, with 26 of 132 species identified outright — more than double what the refractometer manages by itself.

The refractometer is not overrated. It is just the fourth thing to buy, not the first.

One row deserves a note: the polariscope — two polarising filters over a light, with the stone between the filters. It reports whether a stone is singly refractive, doubly refractive or an aggregate, and that one call leaves a median of 75 of the 132 species. Roughly halving the field is real work from two pieces of film, but it narrows less than any measurement below it in the table, which is why it is worth owning and absent from the buying order. The simulated call is also cleaner than the real one: 22 of the 34 singly refractive species can show anomalous double refraction — strain that flickers like double refraction — so reading it right is a technique of its own. How to make the call →

Why cheap and varied beats expensive and precise

A refractometer measures one property extremely well. Specific gravity, magnetic response and fluorescence measure three unrelated properties roughly. Three rough cuts through different dimensions remove more of the field than one sharp cut through a single dimension — because every stone that survives the first cut still has to survive two more that have nothing to do with it.

This is why a stone can defeat an expensive instrument and fall immediately to a cheap one. Two examples straight from the data on this site:

Pyrope and tsavorite garnet have the same refractive index range, 1.730–1.760. A refractometer cannot tell them apart. A magnet can — pyrope responds, tsavorite does not.
Jet and topaz overlap on refractive index (1.640–1.680 against 1.609–1.643) but sit at specific gravity 1.32 and 3.53. A kitchen scale separates them instantly; the expensive instrument hesitates.

Buy in this order

1 · A scale that reads to 0.01 g

Specific gravity by hydrostatic weighing: weigh the stone in air, then suspended in water. It is the cheapest real measurement in gemology and on its own it takes 132 candidates down to a median of 24. It works on rough and irregular stones with no polished facet — the ones a refractometer struggles with. Full method →

2 · A rare-earth magnet — a few dollars

Magnetic response is fast, free once you own the magnet, and separates several look-alike families that optical tests handle badly. Adding it takes the median from 24 to 20. Magnetism reference →

3 · A UV torch

Longwave fluorescence is the third independent axis, and the one that completes the cheap kit at a median of 8. Treat it as corroboration rather than proof: fluorescence varies within a species, which is exactly why it supports an identification instead of deciding one. Fluorescence reference →

4 · Then the refractometer

Now it earns its price. On top of the cheap kit it takes the median to 4 and identifies 26 of 132 species outright. How to use one →

The hard case: stones already set in jewellery

Most people's first specimen is a ring in a drawer, and that is the awkward one. You cannot measure specific gravity on a mounted stone — you would be weighing the metal too — which removes the single most useful cheap test. What still works through a setting is UV, a Chelsea filter and a dichroscope.

Kit (mounted stone)	Median left	Narrowed to ≤5	Tools
Colour + UV	119	0	UV torch
+ Chelsea filter	31	0	+ Chelsea filter
+ dichroscope	28	6	+ dichroscope

Worth being blunt about: 28 candidates for that equipment, and not one of the 132 species identified outright. Mounted stones are genuinely hard, and anyone who tells you a pocket gadget solves them is selling something. If the stone can be unset safely by a jeweller, the cheap kit becomes far more powerful. Working with mounted stones →

How this was measured, and where it overstates

Method. Every one of the 132 species in our database was run through the same identification engine the app uses, once per kit, using that species' published property values as the readings. The figures are the median, the count identified outright, and the count narrowed to five or fewer.

Where it overstates. Every reading is simulated as perfect. Real hydrostatic weighing carries error and fluorescence varies within a species, so these are upper bounds on what a kit achieves, not guarantees. Your kitchen scale will not be perfect, and two specimens of the same species can fluoresce differently. Read these as an upper bound on what a kit can do, not as a promise about your stone.

What colour does. Colour is not in any kit above, and the table shows why: on its own it eliminates nothing, leaving all 132 candidates. A species can occur in several colours and colour shifts with lighting and treatment, so it tells you where to look rather than what to rule out.

The engine that produced this table is the app. Enter what you actually measured — SG, magnetic response, fluorescence, and a refractive index when you have one — and it shows which of the 132 species remain consistent with your readings, and which test would eliminate the most of what is left. It needs real measurements; it cannot identify a gem from a photograph.

Open the app

Common questions

What should I buy first to test gemstones?

A scale that reads to 0.01 g and a way to suspend a stone in water. Specific gravity is the cheapest real measurement in gemology and, measured across 132 species, it narrows the field from 132 to a median of 24 candidates on its own.

Do I need a refractometer to identify gemstones?

Eventually, yes — but not first. Measured across 132 species, a refractometer used alone leaves a median of 22 candidates, while a kit of specific gravity, magnetism and UV fluorescence leaves 8. Added to that cheap kit, a refractometer takes you to 4 and more than doubles the stones identified outright, so it is the right fourth purchase rather than the right first one.

Can I test gemstones that are set in jewellery?

Partly. You cannot measure specific gravity on a mounted stone because you would be weighing the metal too, which removes the single most useful cheap test. UV, a Chelsea filter and a dichroscope all work through a setting, but they narrow far less — a median of 28 candidates, and none of our 132 species was identified outright that way.

Why does a cheaper kit narrow more than a refractometer?

Because they measure independent things. A refractometer is one very precise axis. Specific gravity, magnetic response and fluorescence are three separate axes, and three rough cuts through different dimensions eliminate more than one sharp cut through a single dimension.

Where to go next

Learn gemology free — a 12-lesson course through the technique, no fee or enrolment.
How to measure specific gravity — the first purchase, step by step.
Specific gravity chart — every species, sorted.
The full identification walkthrough.