Gemstone terminology

Aura - a coating treatment that produces an iridescent effect.

Aurora borealis - a coating treatment that produces an iridescent effect.

Buff-top - a type of cut with a smooth, domed top and faceted back.

Cabochon - any shape of cut gemstone which has a flat back and domed (or faceted) top.

Clarity - describes the visual appearance of a gemstone affected by the number, position, size and visibility of any inclusions.

Clarity enhanced - used to describe a gemstone that has been treated to improve its appearance.

Coating - is a layer of material applied to the outer surface of a cut gemstone, as seen on mystic gems for example.

Colour-change - two distinctly different colours (which don’t border each other on the colour wheel) are seen when a gemstone is lit by different light sources such as daylight and incandescent light, for example, the green to red seen in Alexandrite.

Colour-shift - two different colours that are next to each other on the colour wheel are seen when a gemstone is lit by different light sources such as daylight and incandescent light, for example, the pinkish-brown to orange-brown seen in some colour shift garnets.

Composite - gemstones which are artificially assembled from two or more natural, synthetic or artificial components to give the impression of a single gem, for example, opal doublet.

Cultured - pearls where a bead or irritant is deliberately placed in a mollusc to initiate the growth of a pearl.  May also be described as farmed pearls.

Diffusion - a treatment which alters the colour or appearance of a gemstone by introducing chemical elements.  Some sapphires may be treated using this process.

Doublet - a composite gemstone with two different parts that are joined together, for example, an opal doublet.

Dyed - a treatment that artifically introduces colour to a gem material.  May not be a stable treatment that will be long-lasting.  May be seen in any absorbent material or those with surface-reaching fractures (cracks) such as turquoise, pearls, quartz, jadeite.

Facet - a flat surface that has been cut and polished on a gemstone.  

Fire (or dispersion) - flashes of colour displayed when it splits white light into the rainbow colours (think a prism splitting light) as seen in diamonds, moissanite, sphene zircon.

Foiled or foil backed - a coating applied to the back of a gemstone to reflect light back through it.

Fracture filling - a treatment where surface-reaching fractures (cracks) within a gemstone may be filled with oil, resin or glass to improve the appearance of a gemstone.  Many emeralds are treated using the process.

Glass filling - a treatment where surface-reaching fractures (cracks) within a gemstone may be filled with glass to improve the colour and clarity of a gemstone.  Some sapphires and rubies may be treated using this process.

Heat treatment - treatment where a gemstone is subjected to heat to improve its colour or clarity - a practise that has been carried out for thousands of years!  Around 95% of sapphire on the market will have been heat treated.  May also be used to induce an optical phenomenon such as a star in ruby or sapphire.

Hollow back or HB - used to describe a dark-coloured cabochon whose appearance is improved by hollowing out the back of the cabochon, lighting its colour.

Imitation - materials which have a similar appearance to a gemstone but do not share its chemical, physical or optical properties, for example, cubic zirconia imitates diamond.

Impregnation treatment - unstable, soft or porous gemstones may be stabilised or impregnated with a resin to improve their durability so that they are more suitable for use in jewellery.  Turquoise and Blue John fluorite are both typically treated using this process.

Inclusions - internal visual features within a gemstone such as crystals, cavities, bubbles, fractures or healed fractures.

Iridescence - colour/s produced by light being reflected and split by the internal structure of a gemstone, as seen in labradorite for example.

Irradiation treatment - some gemstones may be bombarded by atomic particles to alter the atomic structure and therefore their colour.  Most blue topaz has been treated using this process which is carefully regulated to ensure treated stones possess lower levels of radiation than naturally occurring, background radiation.

Lab-grown/Laboratory grown - manmade gemstones which have been grown in a laboratory/factory.

Lapidary - a person who cuts and polishes gemstones.

Laser drilling - a treatment to improve the appearance of dark inclusions where a tiny laser hole is drilled to the inclusions which is then removed with an acid.  These holes may then be glass filled.

Lead-glass filling - a treatment where surface-reaching fractures (cracks) within a gemstone may be filled with glass to improve the colour and clarity of a gemstone.  Some diamonds, sapphires and rubies may be treated using this process.

Lustre - the surface reflection effect of a gemstone.

Manmade - a material that has been created by man to imitate (eg cubic zirconia) or if synthetic, replicate (eg diamond), a gemstone.

Mystic - a coating treatment that produces an iridescent effect

Natural - I’ve included this as I’ve seen this word used many times to describe materials that are very clearly not…  Study any gemstone closely that is described as natural and ask the seller to explain exactly how and why the stone is natural.  FYI, the dictionary definition says “existing in or derived from nature; not made or caused by humankind”… this seems to allow some wriggle room around treating a natural gemstone (for example, dyeing fossil coral), but I feel the use of natural in this instance is misleading and does not disclose the fact that the stone has been treated…  Buyer beware.

Oiled - a treatment where surface-reaching fractures (cracks) within a gemstone may be filled with oil to improve the appearance of a gemstone.  Many emeralds are treated using this process.

Orient of a pearl - produced by a combination of surface reflection (lustre) & iridescence.

Parti-coloured - a crystal which experienced differing conditions while growing, resulting in one which displays different colours in different parts, for example, watermelon tourmaline.

Paste - artificial (manmade) glass gemstone.

Play of colour - is used to describe the optical phenomenon in opals where the splitting and interference of white light causes the iridescent appearance of patches of differing colours.

Reconstructed or reconstituted - artificial gemstones that have been manmade by bonding or fusing materials together to form a single material.  Some turquoise and coral may be created by this process.

Soudé stones - faceted gemstones which have a layer of coloured glue joining a transparent and colourless top (crown) and bottom (pavilion).  The transparent colourless stones may be quartz, synthetic spinel, sapphire or beryl.  Different colours of glue are used to imitate various gemstones such as emerald and Paraiba tourmaline.

Synthetic - a manmade gemstone, produced in a laboratory/factory, that possesses the same physical,  chemical and optical properties as its natural counterpart, for example, synthetic diamond.  As they are, to all intents and purposes, identical to their natural counterparts it can be very difficult for a consumer to determine the difference.

Treated or treatment - this is the process of artificially altering a gemstone to improve its colour, appearance or physical properties (such as durability).

Triplet - a composite gemstone of three different parts which have been joined together to appear as one, for example, opal triplet.