Adularescence
The shimmer or glow often seen in moonstone.
Allochromatic / Idiochromatic
Describes whether a gemstone’s colour comes from trace elements (allochromatic) or from the main elements in its composition (idiochromatic).
Asterism
A rare star-shaped light effect on a gemstone. Gemstones that have a star effect tend to be cut into cabochons.
Birefringence
The property of a gemstone that splits light into two rays. A gemstone with a very high birefringence sometimes creates a doubled appearance of facets or inclusions.
Brilliance
The sparkle of a gemstone caused by the reflection of light from its interior. Higher brilliance means more sparkle.
Cabochon
A smooth, domed, polished gemstone cut without facets.
Carat (ct)
A unit of gemstone weight equal to 0.2 grams (1 gram = 5 ct). Not to be confused with “karat,” which refers to gold purity.
Chatoyancy / Cat’s Eye
A single line “cat’s eye” effect seen in gemstones with parallel needle-like inclusions. These gemstones are usually cut into cabochons.
Cleavage
The way a gemstone naturally splits along specific planes in its crystal structure. Stones like topaz have perfect cleavage and can break easily if struck.
Colour Zoning
Uneven colour distribution within a gemstone, often seen in sapphires, amethyst, and ametrine.
Dispersion
The splitting of light into spectral colours, creating a “fire” effect. Diamonds are famous for high dispersion.
Dichroism / Pleochroism
The ability of a gemstone to show two (dichroism) or more (pleochroism) colours from different angles. Can be observed with a simple tool called a dichroscope.
Doubly Refractive
A gemstone that splits a ray of light into two beams as it enters, producing two refractive indices. Most, but not all gemstones, and doubly refractive, making this a useful, and quick, test to rule out glass when identifying an unknown stone.
Durability
How well a gemstone withstands everyday wear, considering hardness, toughness, and stability.
Faceted / Facet Cut
A gemstone cut with multiple flat surfaces (facets) to enhance sparkle and brilliance.
Fluorescence
A gemstone’s ability to glow under UV light. Some gemstones change colour as well as glow when lit with a UV lamp.
Fracture
A break in a gemstone that doesn’t follow its natural cleavage planes, which can weaken the stone.
Inclusions
Internal features such as minerals, gas bubbles, or liquid-filled cavities. They can add character, indicate authenticity, or help identify the gemstone.
Labradorescence
The striking play of iridescent colours seen in labradorite, caused by light scattering between layers within the stone.
Lustre
The appearance of light reflected from a gemstone’s surface, described as vitreous (glassy), metallic, silky, pearly, or waxy.
Mohs Hardness Scale
Measures a gemstone’s scratch resistance from 1 (softest) to 10 (hardest). Diamond is 10, talc is 1. Household dust is around 7 due to its quartz content, so stones softer than 7 tend to scratch easily with daily wear.
Optical Phenomena
Special light effects seen in gemstones, including chatoyancy (cat’s eye), asterism (star), and adularescence (moonstone shimmer).
Refractive Index (RI)
How much light bends when entering a gemstone, useful for identification. Doubly refractive gems have two RI values; singly refractive gems have one.
Silk Inclusion
Fine, needle-like inclusions, often seen in sapphire, ruby, and garnet.
Simulant Gemstone
A material that imitates the appearance of a gemstone but is made of a different substance, e.g., cubic zirconia for diamond.
Singly Refractive
A gemstone that does not split light into two rays, having only one refractive index. For Example Garnet, Spinel and Diamond. Glass and cubic zirconia are also singly refractive.
Stability
A gemstone’s resistance to heat, chemicals, and light. Some stones can fade in sunlight, while others remains stable.
Synthetic Gemstones
Lab-grown gems with the same chemical, physical, and optical properties as natural stones, e.g., lab-grown sapphire or ruby.
Tone, Hue, Saturation
Terms describing colour: hue (actual colour), tone (lightness/darkness), and saturation (intensity).
Toughness
How resistant a gemstone is to breaking or chipping. Jade is very tough, while diamond, though extremely hard, can be brittle.
Transparency / Clarity
Measures how much light passes through a gemstone and how free it is from inclusions or blemishes.
Processes to enhance or change a gemstone’s appearance, such as heat treatment, resin filling, or colour coating.
Untreated Gemstones
Stones that have only been cut and polished, with no additional enhancements or treatments.