Start here
How to choose a coloured gemstone
Colour, clarity, cut and carat for coloured stones, plus treatment, origin, durability and certificates.
3 min read
Read the guideGemstone guide
Know the stone
before you choose it
Each gem below has its own page: what gives it colour, how hard it is, where it is mined, how it is usually treated and how to look after it. Start with our buying guide if you are new to coloured stones.
Gemstones A–Z
Amethyst
- Hardness
- Mohs 7
- Colours
- Pale lilac to deep reddish or bluish purple
Aquamarine
- Hardness
- Mohs 7.5–8
- Colours
- Very light blue to deep blue, often with a slight green tint
Chrysoprase
- Hardness
- Mohs 6.5–7
- Colours
- Light mint to intense apple green, from translucent to almost opaque
Citrine
- Hardness
- Mohs 7
- Colours
- Pale lemon yellow through golden yellow to deep orange and reddish brown
Emerald
- Hardness
- Mohs 7.5–8
- Colours
- Yellowish green through pure green to bluish green
Garnet
- Hardness
- Mohs 6.5–7.5
- Colours
- Red, purplish red, orange, yellow, green and pink; rare colour-change stones
Sapphire
- Hardness
- Mohs 9
- Colours
- Blue in every shade, plus pink, purple, yellow, orange, green, colourless and pinkish orange (padparadscha); red corundum is ruby
Topaz
- Hardness
- Mohs 8
- Colours
- Colourless, yellow, orange, pinkish orange ("imperial"), pink, red, light brown and blue
Tourmaline
- Hardness
- Mohs 7–7.5
- Colours
- Every colour, including pink, red, green, blue, yellow, brown and black, often several in one crystal
Glossary
The words gemmologists and jewellers use, in plain English.
A
- Asterism
- A star-shaped pattern of light, usually with four or six rays, seen on the surface of some cabochons. It is caused by light reflecting from needle-like inclusions arranged in different directions, as in star sapphire.
B
- Birefringence
- The difference between the highest and lowest refractive index of a doubly refractive gem. High birefringence makes the back facets look doubled through the table, as in zircon and some tourmaline.
- Brilliance
- The white light returned to the eye from the inside of a cut gem. It depends on the cut proportions and the refractive index.
C
- Cabochon
- A gem cut with a smooth, domed top and no facets. Used for opaque and translucent materials and to show effects such as chatoyancy and asterism.
- Carat
- The unit of weight for gems: one carat equals 0.2 grams (200 milligrams). It measures weight, not size; gems of different density look different in size at the same weight.
- Chatoyancy
- A single band of light, like a cat's eye, that moves across a cabochon as it is turned. It is caused by parallel fibrous or needle-like inclusions.
- Clarity enhancement
- Filling surface-reaching fissures with oil, resin or glass so they are less visible. Common in emerald; it must always be disclosed.
- Cleavage
- The tendency of some crystals to split cleanly along flat planes when struck. Topaz has perfect cleavage, so it needs protection from hard knocks even though it is hard.
- Colour change
- A gem that shows a different colour in daylight and in warm incandescent light, as in alexandrite and some sapphires and garnets.
- Crown
- The upper part of a faceted gem, above the girdle.
- Culet
- The point or small facet at the bottom of a faceted gem's pavilion.
D
- Dispersion
- The splitting of white light into spectral colours, seen as flashes of colour ("fire"). Demantoid garnet has higher dispersion than diamond.
- Doublet
- An assembled stone made of two parts glued together, for example a thin slice of a gem on a glass or quartz base. It must be sold as a doublet.
E
- Extinction
- Dark areas inside a cut gem that stay dark as it moves, usually caused by poor proportions or a very dark tone.
F
- Facet
- A flat, polished surface on a cut gem. The arrangement of facets is the cutting style.
- Fluorescence
- Visible light given off by some gems under ultraviolet light. Strong red fluorescence enhances the colour of some rubies and spinels in daylight.
G
- Girdle
- The outer edge of a cut gem, where the crown meets the pavilion. It is the part held by most settings.
H
- Heat treatment
- Heating a gem to improve its colour or clarity. Common and stable in sapphire, aquamarine and citrine, and usually accepted when disclosed.
- Hue
- The basic colour of a gem, such as blue or green, often with a secondary hue, as in greenish blue.
I
- Inclusion
- A feature inside a gem, such as a crystal, fluid, fracture or growth line. Inclusions can identify a gem, show it is natural, and sometimes indicate its origin.
J
- Jardin
- French for "garden": the inclusions typical of emerald, accepted as part of its character.
L
- Laboratory report
- A document from an independent gem laboratory stating a gem's identity, whether it is natural, any treatments and sometimes its origin. Often called a certificate.
- Loupe
- A small magnifying lens, usually ten times (10x), used to examine gems. Clarity grades for coloured stones are usually judged by eye, then confirmed with a loupe.
- Lustre
- The way light reflects from a gem's surface: vitreous (glassy) for most gems, adamantine for diamond, waxy for some chalcedony.
M
- Mohs scale
- A scale from 1 (talc) to 10 (diamond) that ranks resistance to scratching. Gems below 7 are scratched by dust, which contains quartz, and dull faster in daily wear.
O
- Origin
- The country or deposit a gem was mined in. Some origins add value, but only a laboratory can determine origin reliably.
P
- Pavilion
- The lower part of a faceted gem, below the girdle. Its angles largely decide how much light returns to the eye.
- Pleochroism
- Different colours or shades seen when a gem is viewed from different directions. Strong in tourmaline, tanzanite and iolite, so cutters orient these stones carefully.
R
- Refractive index
- A measure of how much light slows and bends when it enters a gem. Measured with a refractometer, it is one of the main ways to identify a gemstone.
- Rough
- Gem material that has not yet been cut or polished.
S
- Saturation
- The strength or purity of a gem's colour, from greyish or brownish to vivid.
- Specific gravity
- The density of a gem compared with water. It explains why stones of the same carat weight differ in size.
- Synthetic
- A gem grown in a laboratory with the same chemistry and structure as its natural counterpart. Synthetics must be disclosed and are worth much less than natural gems.
T
- Table
- The large, flat facet on top of a faceted gem.
- Tone
- How light or dark a gem's colour is. Most gems look best at medium to medium-dark tone.
W
- Windowing
- A washed-out, see-through area in the centre of a cut gem, caused by a pavilion that is too shallow to reflect light back.