The short answer
For coloured gemstones, colour matters most: look for a rich, even hue that is neither too dark nor too pale. Clarity standards depend on the stone: emeralds are expected to show inclusions, while blue sapphires and aquamarines are usually expected to be cleaner. A good cut returns light evenly across the stone, without a dull, see-through centre (a “window”). Carat is weight, not size: denser stones look smaller per carat. Always compare stones side by side, in daylight, and with the same report details.
Identity first, then quality
Quality only matters once you know what the stone is. A beautiful blue stone may be a natural sapphire, a laboratory-grown sapphire, a heat-treated sapphire or a glass imitation, and those are priced very differently. Start with identity and treatment, ideally confirmed by an independent laboratory report, then judge quality.
Colour: the biggest factor
Gemmologists describe colour with three parts.
| Part | What it means | What to look for |
|---|---|---|
| Hue | The basic colour: blue, red, green, yellow and so on | A pure hue, with little grey or brown, is usually valued more. A slight secondary colour (for example, violet in a blue sapphire) can be acceptable |
| Tone | How light or dark the colour is | Medium to medium-dark is usually prized. Very dark stones look black in low light; very light stones look washed out |
| Saturation | How strong or vivid the colour is | Strong, vivid saturation is valued. Greyish or brownish colour lowers value |
Practical tips:
- Look at the stone in daylight near a window, then under warm indoor light. Some stones change noticeably.
- Look at it from the top (face up), as it will be worn. Colour seen from the side can be misleading.
- Watch for colour zoning: bands or patches of stronger and weaker colour. Mild zoning is common; strong zoning visible face up lowers value.
- Be careful with photos. Lighting, editing and screens change colour. Ask for an unedited video in daylight.
Clarity: depends on the stone
Inclusions are tiny crystals, fluids, fractures or growth features inside a stone. Some are signs of natural growth, and some can help a laboratory identify a stone. Each type of gem has its own normal clarity.
| Typical clarity | Examples | What buyers usually accept |
|---|---|---|
| Usually clean to the eye | Aquamarine, topaz, citrine, blue sapphire (many qualities) | Eye-clean stones are expected; visible inclusions lower value noticeably |
| Usually some inclusions | Ruby, many sapphires, garnet, peridot, tourmaline (some colours) | Minor inclusions are normal; look for none that are distracting face up |
| Usually included | Emerald, red tourmaline | Inclusions are expected; avoid ones that break the surface or look like cracks |
What to avoid in any stone:
- Surface-reaching fractures, which can weaken the stone and may spread.
- Inclusions under the table (the large top facet) that are obvious face up.
- Unexpected clarity for the price. A large, perfectly clean emerald at a low price deserves questions about treatment or identity.
Cut: how the stone uses light
In coloured stones, cut is not graded as strictly as in diamonds, but it still has a large effect on beauty.
- Brightness and life. A well-cut stone looks bright across most of its surface as you tilt it.
- Windows. A pale, see-through area in the centre usually means the stone is cut too shallow, often to keep weight.
- Extinction. Large dark areas that do not change as you tilt the stone mean light is being lost.
- Proportions. A very deep stone carries hidden weight in the bottom (pavilion). It costs more per visible millimetre and can be hard to set.
- Symmetry and polish. Facets should meet neatly; the outline should be even; the surface should not look hazy or scratched.
Many coloured stones, especially from India and Sri Lanka, are cut to keep as much weight as possible. That is normal, but a slightly smaller, better-cut stone often looks better than a larger, poorly cut one.
Carat weight versus size
A carat is a unit of weight: 0.2 grams. Stones of the same carat weight can be different sizes, because gemstones have different densities.
| Stone | Relative density | Effect at the same weight |
|---|---|---|
| Emerald | Lower | Looks larger per carat |
| Amethyst, citrine (quartz) | Lower | Looks larger per carat |
| Sapphire, ruby | Higher | Looks smaller per carat |
| Zircon | Higher | Looks smaller per carat |
So compare stones by dimensions in millimetres as well as carat weight. A report should list both.
In India, coloured stones are also traded in ratti. A ratti is not one fixed weight everywhere; different sellers use different conversion factors. If a price is quoted per ratti, ask which conversion is being used and ask for the weight in carats as well.
Shape and origin
- Shape. Ovals and cushions are the most common shapes for coloured stones because they keep weight from the rough. Rounds, emerald cuts and other shapes can cost more per carat.
- Origin. Some origins carry a premium. An origin statement is only meaningful when it comes from a laboratory report; a seller’s description of origin is a claim.
Comparing two stones side by side
Use the same steps for every stone you compare.
- Identity and treatment: same type of report, same treatment status?
- Colour: compare in daylight, face up, against a white background.
- Clarity: anything distracting face up? Any surface-reaching fractures?
- Cut: tilt both stones. Which one stays bright? Any window?
- Size: compare millimetre dimensions, not only carats.
- Price: compare price per carat only between stones of similar identity, treatment and quality.
A quality checklist
- Identity and treatment stated, ideally with an independent report
- Colour checked in daylight, face up, in an unedited video or in person
- Clarity normal for that type of stone, with no surface-reaching fractures
- Bright across the face, without a large window or dark areas
- Weight in carats (and ratti conversion, if quoted) and dimensions in millimetres
- Return terms in writing
FAQ
Is a darker stone always better? No. Very dark stones lose brightness and can look black indoors. Medium to medium-dark is usually best.
Are inclusions bad? Not always. Some are expected in certain stones and can confirm that a stone is natural. Avoid inclusions that are distracting or that weaken the stone.
Why does my stone look different from the photo? Lighting, cameras, editing and screens all change colour. Always judge in daylight when you can.
Is a bigger carat weight always better value? No. A poorly cut stone can carry hidden weight. Compare dimensions and appearance as well.
Does a high price mean high quality? Not necessarily. Compare stones using the same steps and similar reports.
Next steps
Read our gemstone buying guide on natural, lab-grown and treated stones and on laboratory reports, browse more Gemstones articles, or email us.