Gemological certification provides an authoritative assessment of a gemstone’s quality and characteristics. Whether you are buying, selling, or simply interested in precious stones, understanding the terminology used in gemological certification is crucial for making informed decisions. This glossary will guide you through key terms related to the certification process, ensuring that both beginners and experts can better appreciate the artistry, science, and precision involved in gemstone evaluation.
Abrasion
Small scratches or wear on the surface of a gemstone, typically seen in diamonds due to contact with other stones or materials.
Absorption Spectra
A graph that displays how a gemstone absorbs light across visible wavelengths. This helps identify the stone’s variety and colour origin.
Annealing
A heat treatment process is used to enhance the appearance or durability of gemstones, especially diamonds. It can be combined with other treatments like high-pressure, high-temperature (HPHT) treatment.
Assembled Stone
A composite gemstone is created by bonding different materials, which may include natural or synthetic gemstones and glass.
Asterism
A star-shaped reflection of light visible on the surface of certain gemstones, like star sapphires, caused by aligned inclusions.
Baguette
A rectangular step-cut is used mainly for diamonds, producing a sleek, geometric appearance.
Baroque
Refers to irregularly shaped gemstones or pearls, particularly in pearls that are non-symmetrical.
Bi-Coloration
The presence of two distinct colours within a single gemstone is often seen in certain types of tourmaline.
Bleaching
A treatment process is used on gemstones and pearls to lighten their colour or improve consistency by removing undesired hues.
Blemish
An external imperfection is found on the surface of a gemstone, such as a scratch, nick, or abrasion.
Brilliance
Refers to the brightness or light return of a gemstone, particularly diamonds, when viewed from above. It is a critical factor in diamond grading.
Brilliant Cut
A diamond or gemstone cut featuring multiple facets arranged to maximise the stone’s sparkle and brilliance. The round brilliant is the most common form.
Bruise
A small area of damage where the surface of a gemstone has been indented or broken.
Bruted Girdle
A girdle that has been cut but not polished, is often seen in diamonds as a sign of rough craftsmanship.
Cabochon
A gemstone that has been polished to create a domed surface without facets, typically used in opaque stones like opals or turquoise.
Carat Weight
A unit of weight for diamonds and gemstones, where one carat equals 0.2 grams or 100 points. It directly impacts the value of the stone.
Cavity
An open flaw or gap in a gemstone that breaks through to the surface.
Chatoyancy
An optical phenomenon in gemstones like cat’s eye chrysoberyl, where a narrow band of light moves across the surface.
Chemical Vapor Deposition (CVD) Diamond
A synthetic diamond is grown using chemical vapor deposition, where carbon atoms are layered to form a diamond.
Chip
A small break or shallow surface damage on a gemstone, often near the edges.
Clarity Grade
A rating system that evaluates the number, size, and visibility of internal inclusions and external blemishes in a diamond. Ranges from FL (Flawless) to I (Included).
Cleavage
A weak area in a gemstone’s structure can break along certain planes under stress.
Cloud
A group of minute inclusions that form a hazy or cloudy area inside a gemstone, affecting clarity.
Colour Grade
An assessment of a diamond’s colour, typically ranging from D (colourless) to Z (light yellow or brown). Coloured diamonds are graded separately based on intensity.
Conflict Diamond
A diamond mined in war zones and sold to finance insurgencies, a subject of international concern. Conflict-free diamonds are certified through the Kimberley Process.
Crown
The upper part of a faceted gemstone, above the girdle, includes the table (top flat facet).
Culet
The small, flat facet at the bottom of a gemstone, is typically found in diamonds. A larger culet can affect the gemstone’s brilliance.
Cultured Pearl
A pearl is created by inserting a nucleus into an oyster, which then coats the nucleus with layers of nacre. Common types include Akoya, Tahitian, and South Sea pearls.
Cut Grade
A rating that assesses the quality of a gemstone’s cut, considering its proportions, polish, and symmetry. Excellent cuts maximise the stone’s light performance.
Dispersion
The splitting of white light into spectral colours (fire) as it passes through a gemstone. High dispersion is valued in diamonds for their brilliance.
Durability
A combination of a gemstone’s hardness, toughness, and stability, indicates its ability to withstand wear and tear over time.
Feather
An internal crack or fissure within a gemstone may affect clarity and durability.
Fluorescence
The ability of certain gemstones to glow under ultraviolet light, which can impact a diamond’s appearance under certain lighting conditions.
Four Cs
The four key attributes used to assess a diamond’s quality: Carat, Clarity, Colour, and Cut.
Fracture Filling
A treatment used to improve the clarity of a gemstone is filling cracks with a glass-like substance.
Girdle
The narrow band that separates the crown from the pavilion is a faceted gemstone, which can be bruted, polished, or faceted.
Graining
Visible lines or patterns within a diamond or gemstone, resulting from its crystal structure.
Inclusion
An internal flaw, such as a crystal or crack, within a gemstone. Inclusions affect the clarity grade and value.
Kimberley Process (KP)
An international certification scheme that ensures diamonds are sourced from conflict-free areas.
Lab-Grown Diamond
A man-made diamond created in a controlled environment. It has the same chemical and physical properties as a natural diamond.
Laser Drilling
A process that uses a laser to create tiny holes in a diamond to reach and treat inclusions, improving clarity.
Lustre
The quality of light reflection on the surface of a pearl or gemstone, contributing to its beauty.
Melee
Small diamonds or gemstones, usually weighing 0.12 carats or less, are often used as accents in jewellery.
Mohs Scale
A scale of mineral hardness ranging from 1 (talc) to 10 (diamond), is used to assess the scratch resistance of gemstones.
Nacre
The iridescent substance produced by oysters that form pearls. The thickness of nacre affects a pearl’s durability and lustre.
Pavilion
The lower portion of a faceted gemstone, below the girdle, influences how light is reflected through the crown.
Plotting Diagram
A schematic representation of a gemstone’s inclusions and blemishes is used in diamond grading reports to indicate the location and type of imperfections.
Proportions
The measurements and angles of a gemstone’s facets, which determine its overall cut quality and light performance.
Saturation
A measure of a gemstone’s colour intensity, affects its overall value and appearance.
Scintillation
The sparkling effect created by a gemstone when it is moved in the light, caused by reflections off its facets.
Transparency
The degree to which light passes through a gemstone. It can range from transparent to opaque.
Twinning Wisp
A ribbon-like inclusion formed during crystal growth, common in diamonds.
