Buying a colored gemstone comes down to two jobs. First, judge the stone in front of you: its color, clarity, cut and size. Second, confirm what it is: the species, whether it is natural or lab-grown, and whether it has been treated. Colored stones have no single grading scale like the one used for diamonds, so the second job depends heavily on disclosure and, for expensive stones, an independent lab report.
Start with color
Color sets most of a colored gem's value. Gemologists describe it with three terms: hue (the main color, such as blue or violetish blue), tone (how light or dark it is) and saturation (how strong or grayish the color looks). The most prized stones usually show a pure hue, a medium to medium-dark tone and strong saturation. A sapphire that is too dark looks inky in dim light, and one that is too light looks washed out.
Look at the stone in more than one kind of light. Daylight, warm indoor bulbs and the bright LEDs in jewelry cases all shift color. Some gems change on purpose: alexandrite turns from green or bluish green in daylight to red or purple under incandescent light. Others, such as some tanzanite, shift between blue and violet. The color in gemstones page explains what causes these shifts.
Also check color zoning. Sapphire and amethyst often show bands of stronger and weaker color. A good cutter orients the stone so the zoning is hidden when you look at it face-up.
Clarity expectations depend on the gem
Each gem species has its own normal level of inclusions, so clarity has to be judged against that species. The trade often sorts gems into three loose groups:
- Gems usually found eye-clean, such as aquamarine, topaz, citrine and blue zircon. Visible inclusions in these lower the price noticeably.
- Gems that often contain some inclusions, such as ruby, sapphire, garnet, peridot and most tourmaline. Minor inclusions are accepted, especially if they are hard to see without magnification.
- Gems that almost always show inclusions, such as emerald and red tourmaline (rubellite). Here the question is whether the inclusions hurt the look or the durability of the stone.
Watch for surface-reaching fractures, which can weaken a stone, and for large inclusions under the table, which are most visible. Some inclusions are welcome: fine rutile needles cause the star in star sapphire, and certain inclusions help labs determine origin. The colored gemstone grading page covers the clarity types in more detail.
Judge the cut
Colored stones are often cut to save weight, since the rough is priced by the carat. That can leave a stone with a deep, bulging pavilion that adds weight without adding beauty, or a shallow pavilion that leaves a "window," a washed-out area in the center where you can see straight through the stone.
Hold the stone face-up and tilt it. A well cut gem shows even color and lively light across most of its face, with little dark extinction and no large window. Check symmetry too: the outline should be balanced, and facet junctions should meet neatly. For an overview of shapes and facet styles, see gemstone cut shapes and styles.
Carat weight and size
Gem prices usually rise faster than weight, because large fine stones are rarer than small ones. A two-carat stone often costs well over twice as much per carat as a one-carat stone of the same quality.
Remember that carat is a weight, not a size. Gems differ in density: a one-carat ruby looks smaller than a one-carat emerald because corundum is heavier. If face-up size matters to you, compare millimeter measurements as well as weight. The specific gravity page explains why.
Ask about treatments
Most colored gems on the market have been treated in some way. Heating of ruby and sapphire is widespread and permanent. Oiling of emerald is routine but can change over time. Blue topaz is nearly always irradiated. Other treatments, such as lead-glass filling of ruby or beryllium diffusion of sapphire, have a large effect on price and care.
Under the U.S. Federal Trade Commission's Jewelry Guides, sellers should disclose a treatment when it is not permanent, when it creates special care requirements, or when it affects value. That covers most treatments. Ask the seller directly, and get the answer in writing on the invoice. A detailed list of methods by gem is on the gemstone treatments page.
Natural, synthetic or imitation
Three kinds of material can look alike:
- A natural gem formed in the earth.
- A synthetic (lab-grown) gem has the same chemistry, crystal structure and properties as its natural counterpart but was made in a factory. Lab-grown ruby, sapphire, emerald, spinel, alexandrite and quartz are widely available.
- An imitation (simulant) only looks like the gem. Glass, plastic, and cubic zirconia are common imitations, along with doublets and triplets built from layers.
Lab-grown gems are real gem material and can be lovely, but they should cost much less than natural stones of similar appearance. The FTC says a lab-made stone sold under a gem name must carry a clear qualifier such as "laboratory-grown," "laboratory-created" or "synthetic." Telling these apart usually takes gemological testing; the how to identify gemstones guide explains what labs look for.
When to get a lab report
For an inexpensive stone, a reputable seller's written description is often enough. For a costly gem, ask for a report from an independent gemological laboratory. A colored stone report typically states the species and variety, whether the stone is natural or synthetic, detectable treatments, measurements and weight. Some labs also offer an opinion on geographic origin, which can matter a lot for ruby, sapphire and emerald.
A lab report describes the stone and does not state a price. An appraisal, prepared by an appraiser, gives a value estimate for insurance or resale. Read more about how reports work in the diamond certificates guide, since the format of colored stone reports is similar.
Seller policies and paperwork
Before paying, check the following:
- A written invoice naming the gem, its weight, any treatments and whether it is natural or lab-grown.
- A return period long enough to have the stone checked by an independent appraiser or lab.
- The seller's policy if a lab later finds an undisclosed treatment or a synthetic.
- For online purchases, clear photos and video in several lighting conditions and the exact dimensions.
Once the stone is set, choose a mounting that suits its hardness. The gemstone durability guide covers which gems are safe for daily wear, and gemstone settings explains which settings protect softer stones.
Frequently asked questions
Is a heated sapphire or ruby worth less?
Heating is accepted and very common, so heated stones make up most of the market. Unheated stones of the same quality usually sell at a premium, and a lab report is the normal way to confirm no heat.
Are lab-grown gemstones fake?
No. A lab-grown ruby is corundum, the same as a natural ruby. It is fake only if it is sold as natural, which is why disclosure matters.
Do I need a lab report for every gemstone?
Not for low-cost stones. For an expensive gem, or any stone where origin or lack of treatment is part of the price, an independent report is a sensible safeguard.
How can I tell if a gemstone has been treated?
Most treatments cannot be seen without lab equipment. Ask the seller in writing and, for expensive stones, rely on a report from a gemological laboratory.