Light & appearance
Asterism
A star-shaped glow that appears when light reflects from aligned needle-like inclusions inside a gem.
See it in rubyGemstone Glossary
Build your gemology vocabulary with straightforward definitions for the terms used in gemstone buying, grading, jewelry, and mineral collecting.
Gemology, in plain language
These definitions introduce the terms used throughout the gemstone profiles. They are not a substitute for a laboratory report or professional appraisal.
Gemstone glossary terms
Light & appearance
Color zoning
Visible areas of different color or intensity within one gem, formed as the crystal grew.
See it in tourmalineLight & appearance
Dispersion
The splitting of white light into rainbow flashes, often called a gem’s fire.
See it in garnetLight & appearance
Fluorescence
A glow some gems give off when exposed to ultraviolet light. It can vary from stone to stone.
See it in diamondLight & appearance
Luster
The way a gem’s surface reflects light, such as glassy, silky, pearly, or metallic.
Compare gem profilesLight & appearance
Play-of-color
Shifting flashes of spectral color seen as an opal moves in the light.
See it in opalLight & appearance
Pleochroism
A gem showing different colors, or different strengths of color, when viewed from different directions.
See it in tanzaniteLight & appearance
Transparency
How easily light travels through a gem: transparent, translucent, or opaque.
Compare gem profilesStructure & durability
Cleavage
A natural direction in a crystal where it can split more easily after a sharp impact.
See the care notes for tanzaniteStructure & durability
Inclusion
A mineral, fluid, or tiny feature trapped inside a gem as it forms. Inclusions can help identify a stone.
See it in emeraldStructure & durability
Mohs hardness
A 1-to-10 scale describing how well a gem resists scratches. It does not measure toughness against breaking.
Compare hardness in profilesTreatments & construction
Fracture filling
A treatment that fills surface-reaching cracks with a glass-like material to make them less visible.
Read diamond treatment notesTreatments & construction
Hydrophane
A porous gem material that can absorb water, sometimes changing its appearance or transparency temporarily.
See it in opalTreatments & construction
Doublet & triplet
A layered gem made from two or three pieces, often with a thin color layer joined to a backing or clear cap.
Read opal buying guidanceIdentification
Refractive index
A number describing how strongly a gem bends light. Gemologists use it as an important identification clue.
See the science notes for diamondIdentification
Specific gravity
A gem’s density compared with water. It helps distinguish materials that may look alike.
Explore gemstone propertiesIdentification
Treatment
A human process used to improve a gem’s color, clarity, or durability. Reputable sellers should disclose it.
Read treatment notes in profiles