A Jeweller’s Guide to Gemstone Terminology
When working with gemstones, knowing the right terminology is key. Whether you’re selecting stones for a new design, assessing quality, or discussing options with a supplier. This glossary covers the most important gemstone terms to help you navigate the world of gems with confidence.
Quick side note - Precious vs. Semi-Precious
Traditionally, gemstones were divided into “precious” (diamond, ruby, sapphire, and emerald) and “semi-precious” (everything else). However, this distinction is misleading. Some so-called “semi-precious” stones, like a fine-quality spinel or Paraíba tourmaline, can be more expensive than sapphires, as such these terms are generally being phased out by the industry.
Gemstone Glossary
Birefringence – The optical property where a gem splits light into two rays, creating a doubling effect in some stones like zircon and peridot. It refers to the difference between the two principal refractive indices (i.e. it's the difference between the two RI readings of a doubly refractive gemstone).
Brilliance – The amount of light reflected from a gemstone’s interior. The higher a gemstones brilliance, the more sparkly it will be.
Cabochon – A smooth, domed, and polished gemstone cut, rather than a faceted one.
Carat (ct) – A unit of weight for gemstones, equal to 0.2 grams (there are 5 carats to a gram). Not to be confused with “karat,” which refers to gold purity.
Cleavage – The way a gemstone naturally splits along certain planes due to its atomic structure. Stones with perfect cleavage, like topaz, need careful handling as they will easily split if struck.
Colour Zoning – Uneven colour distribution within a gemstone, often visible in sapphires and amethyst and the effect that causes ametrine to exist.
Dispersion – The splitting of light into spectral colours, creating “fire.” Diamonds have high dispersion, producing a rainbow-like sparkle.
Dichroism / Pleochroism – The ability of a gem to show two (dichroism) or more (pleochroism) different colours when viewed from different angles, seen in stones like iolite and andalusite. Pleochroism can be easily observed using a simple tool called a dichroscope.
Doubly refractive - A term used to describe a gemstone that splits a ray of light into two beams as the light enters it.
Durability – A combination of hardness, toughness, and stability that determines how well a gemstone withstands wear and environmental factors.
Faceted Cut – A gemstone cut with multiple flat surfaces (facets) to enhance brilliance and sparkle.
Fluorescence – A gemstone’s ability to glow under UV light, commonly seen in some diamonds and rubies.
Fracture – A break in a gemstone that does not follow its natural cleavage planes, often affecting durability.
Inclusions – Internal features within a gemstone, such as minerals, gas bubbles, or liquid-filled cavities. Some inclusions can add character, prove authenticity, or help with gemstone identification.
Lustre – The way light reflects off a gemstone’s surface, described based on how it looks, for example vitreous (glassy), metallic, silky, pearly, or waxy.
Mohs Hardness Scale – Sometimes refereed to simply as a gems hardness, the Mohs hardness scale goes from 1 to 10 measuring a gemstone’s scratch resistance. Diamond is the hardest at 10, while talc is the softest at 1.
(My favourite side note - household dust has a hardness of 7 due to it containing small particles of quartz, which is why a gem with a hardness of less than 7 isn't ideal for jewellery piece designed for everyday wear.)
Optical Phenomena – Unique light effects in gemstones, including chatoyancy (cat’s eye effect), asterism (star effect), opalescence (a milky glow), and adularescence (moonstone shimmery glow).
Refractive Index (RI) – A measure of how much light bends as it enters a gemstone, very helpful when determining a gem’s identity.
(Doubly refractive gems have two RI's due to the split light beam bending in different directions, where as singly refractive gemstones only have one.)
Silk – Fine needle-like inclusions within gemstones, often seen in sapphire and ruby. It is also usually responsible for the cats eye (Chatoyancy) and star (Asterism) optical effects.
Simulant Gemstone - An imitation of a gemstones appreance, but made from a different material. For example blue coloured glass being used as a simulant sapphire, or cubic zirconia as a simulant diamond.
Singly refractive - A term used to describe a gemstone that doesn't splits a ray of light.
Stability – A gemstone’s resistance to heat, chemicals, and light exposure. Some gems, like kunzite, fade in sunlight, while others, like quartz, are highly stable.
Synthetic Gemstones – Man-made/lab grown gemstones with the same chemical, physical, and optical properties as their natural counterparts. For example lab grown sapphires or rubies are synthetic gemstones.
Toughness – A gemstone’s resistance to breaking or chipping. Jade is one of the toughest gemstones, even though it is not particularly hard, while diamond, despite being the hardest gemstone, can still be brittle due to its perfect cleavage.
Treatments – Processes used to improve or change a gemstone’s appearance, such as heat treatment for sapphire, resin filling for emerald or colour coating for mystic topaz.
Ultrasonic Cleaner – A device that cleans gemstones and jewellery using high-frequency sound waves. Not all gemstones are safe for ultrasonic cleaning, especially porous or fracture-filled stones which should be cleaned with care.
Untreated Gemstones – Gemstones that have not undergone any enhancements or treatments beyond cutting and polishing.
Understanding gemstone terminology helps jewellers make informed decisions when selecting, buying, and working with stones. Whether you're sourcing an untreated sapphire, assessing a gem’s durability for a ring, or admiring the chatoyancy of a cat’s eye chrysoberyl, these terms will help you speak the language of gemstones with confidence!