Gemologists identify a stone by measuring its physical and optical properties and comparing them with known values. Color is the first thing most people notice, but it is one of the least reliable clues, because many different minerals share the same colors. A confident identification usually combines several tests: refractive index, specific gravity, optic character, the absorption spectrum and a look at the inclusions under magnification.
Why color is not enough
A red stone could be ruby, spinel, garnet, red tourmaline, glass or a synthetic ruby. Several famous "rubies" in European crown jewels, including the Black Prince's Ruby in the British Imperial State Crown, turned out to be spinel once gemologists could measure their properties. Blue stones raise the same problem: sapphire, tanzanite, iolite, kyanite, blue topaz, aquamarine and glass can all look similar in a case.
Many gems also come in a wide range of colors. Sapphire can be blue, pink, yellow, green or colorless. Tourmaline covers nearly the whole spectrum. So color narrows the list of possibilities without settling it. The color in gemstones page explains why some minerals have fixed colors and others vary.
Refractive index
The refractive index (RI) measures how much a material slows and bends light. Each gem species has a narrow, well-documented RI range, which makes it one of the most useful single tests. Gemologists measure it with a refractometer, an instrument where the polished stone sits on a glass prism with a drop of contact liquid.
Some typical readings:
| Gem | Refractive index (approx.) |
|---|---|
| Quartz (amethyst, citrine) | 1.544-1.553 |
| Beryl (emerald, aquamarine) | 1.57-1.59 |
| Tourmaline | 1.62-1.64 |
| Spinel | 1.718 |
| Corundum (ruby, sapphire) | 1.762-1.770 |
| Diamond | 2.42 |
A standard refractometer only reads up to about 1.81, the limit set by the contact liquid. Diamond, zircon, cubic zirconia and other high-RI materials read "over the limit" and need other tests. The refractive index guide lists many more values.
Optic character and birefringence
Gems split into two optical groups. Singly refractive (isotropic) materials, such as diamond, spinel, garnet and glass, send light through at one speed in every direction. Doubly refractive (anisotropic) materials, which include most other gems, split light into two rays traveling at different speeds.
On a refractometer, a doubly refractive gem shows two readings, and the difference between them is its birefringence. Corundum shows about 0.008, while peridot shows around 0.036. In strongly birefringent stones such as zircon and peridot, you can see the back facet edges doubled through a loupe. A polariscope tells singly and doubly refractive stones apart quickly. See birefringence and double refraction for how this works.
Doubly refractive colored stones may also show pleochroism, different colors seen from different directions. A dichroscope reveals it. Tanzanite and iolite show strong pleochroism, while garnet and spinel show none. The pleochroism page has more examples.
Specific gravity
Specific gravity (SG) compares a stone's density with that of water. Gemologists find it by weighing the stone in air and again suspended in water, or by seeing whether it floats or sinks in heavy liquids of known density. Quartz is about 2.65, beryl about 2.7, topaz about 3.5, corundum about 4.0 and cubic zirconia about 5.6 to 6.0.
SG is handy for loose stones and for separating look-alikes. Topaz and citrine can share a color, but topaz is far denser. The method does not work on mounted stones. More values are in the specific gravity table.
Absorption spectrum
A handheld spectroscope spreads white light passing through or reflecting from a stone into a rainbow. Dark lines or bands show which wavelengths the stone absorbs, and these patterns are tied to the elements causing the color. Chromium in ruby, red spinel and emerald produces fine lines in the red end. Iron in almandine garnet and blue sapphire produces bands in the green and blue. An experienced gemologist can sometimes identify a stone, or confirm a dye, from its spectrum alone.
Inclusions under magnification
A 10x loupe or a gemological microscope often gives the clearest answer. Natural gems grow slowly in the earth and trap minerals, fluids and growth features typical of their species and source. A Colombian emerald may hold three-phase inclusions with a crystal, a gas bubble and liquid in one cavity. Demantoid garnet can show fine "horsetail" fibers. Natural sapphire often shows angular color zoning and fine rutile silk.
Lab-grown and imitation materials have their own signs. Flame-fusion synthetic ruby and sapphire can show curved growth lines and gas bubbles. Glass often has round bubbles, swirl marks and molded, rounded facet edges. The gemological instruments page describes the microscope and other tools.
What you can and cannot do at home
A loupe, a good light and patience go a long way. At home you can check for obvious glass features, look for doubling of back facets, spot doublets by a join line at the girdle, and note scratches or worn facet edges that point to a soft material. Scratch tests on a stone you care about are a bad idea, since they can damage the gem and often give misleading results.
Home observation cannot separate natural from lab-grown corundum, confirm most treatments, or identify an unknown stone with certainty. Thermal and electrical testers are designed for diamond and its simulants and say little about colored stones. For diamonds, see how to tell if a diamond is real.
When to use a gemological lab
Send a stone to a lab when it is expensive, when you are buying or selling, or when treatment or origin affects its value. Labs use standard gemological tests plus advanced equipment such as FTIR and Raman spectroscopy, UV-visible spectroscopy and laser ablation mass spectrometry (LA-ICP-MS), which measures trace elements. These methods detect heat treatment, beryllium diffusion, glass filling and most lab-grown material. For buying advice built around these checks, see how to buy a gemstone.
Frequently asked questions
Can I identify a gemstone by its color?
Not reliably. Many species share the same colors, and many single species come in several colors, so color only narrows the options.
What is the single most useful test?
Refractive index is often the most informative quick test for a polished colored stone, but gemologists confirm it with other tests before naming the gem.
Can a jeweler tell a synthetic from a natural gem?
Sometimes, by spotting growth features under magnification. Many modern lab-grown gems need advanced lab testing to confirm.
Does the scratch test work?
It can damage the stone and gives unreliable results on cut gems. Gemologists do not use it on finished stones.