Physical and Optical Properties of Gemstones

Learn about gemstone color, crystallography, hardness, chemical makeup and more.

Physical and Optical Properties of Gemstones

  • Figure 2. Color solid. (From Precise Color Communication: Color Control from Feeling to Instrumentation, p.9; courtesy of Minolta Camera Company, Ltd. Japan.)
    Gemstone Color Measurements and Specifications

    One of the great challenges of gemology is the development of an accurate, simple, consistent, and reproducible technique for gemstone color measurements and specifications. Until the mid-80s little progress was made, despite the appearance and…

  • crystallography - bladed baryte
    Descriptive Crystallography for Gemologists

    Gemologists deal with many crystalline minerals. Learn the most common terms used in crystallography to describe their appearance and structure.

  • gem color - GIA hues
    Evaluating Gem Color: Hue, Tone, and Saturation 

    Accurately describing gem color is critical to determining a gemstone’s value. Learn how to grade hue, tone, and saturation and avoid some common pitfalls.

  • Àíäðàäèò
    What Creates Gemstone Color?

    Gemstone color owes a little to pigments and a lot to light. Some gems get their color the same way paints receive their color.  For example, if you wanted some blue paint for your bedroom, the store clerk would take white paint and add a few drops…

  • Gemstone magnetism - ruby and spinel
    Gemstone Magnetism: The Forgotten Property 

    Step 3: Practical Gemology Lesson 18 Back to syllabus Some time ago, I became interested in gemstone magnetism. While researching this property, I found that some gemology schools had little to say on the subject. (Many gemology…

  • “4E2A7069s” by Jared Tarbell is licensed under CC By 2.0
    Optics in Gemology

    Optical properties of gems are incredibly important in gemstone differentiation and identification. Because the optical properties of a crystal are sensitive to minute changes in composition and strain within the crystalline structure, they are…

  • iris agate - phenomenal gems
    Phenomenal Gems – Unique Phenomena in Gemstones 

    Step 5: Advanced Gem Information Lesson 3 Back to syllabus What Are Phenomenal Gems? Some optical effects in gemstones, such as color, brilliance, dispersion, and birefringence, can be quite impressive. However, not all optical…

  • Stone feels cold to the touch because it has a high thermal inertia.  Thermal inertia is of the thermal properties that gemologists can use for gemstone identification.  “Coloured marble cold stone” by Pia Poulsen is licensed under CC By 2.0
    Thermal Properties of Gems

    Gemologists are severely handicapped in analyzing and identifying gemstones because their testing methods must be nondestructive. This limits the measuring process to the areas of optics, including spectroscopy, luminescence, and so forth;…

  • gemstone enhancements - irradiated topaz
    What are Gemstone Enhancements?

    Step 2: Advanced Gemology Lesson 5 Back to syllabus Gemstone enhancements are procedures applied to gems to improve their appearance and wearability. There are many kinds of treatments. Some have been used for centuries, while…

  • askjeff-images-color_wheel1
    Understanding Color in Gemology

    The color of a mineral or gemstone is one of the primary attributes used for identification, but it is, unfortunately, one of the least diagnostic and useful (except in a handful of cases). The colors reported include all those mentioned in the…

  • 1024px-Apatite,_quartz,_orthose,_muscovite_sous_UVL
    What Is Gemstone Luminescence?

    If you’ve ever seen a gem that appeared to be glowing, you’ve witnessed gemstone luminescence. This phenomenon occurs when electrons in certain atoms of a crystallized mineral absorb energy and then release it in small amounts over time. If the…

  • Fluorite - gemstone cleavage
    What is Gemstone Cleavage?

    Just as wood is easier to split with the grain than against it, gemstone cleavage is the tendency of certain crystals to break along definite plane surfaces. If there are planes in a crystal structure with relatively weak atomic bonds, the…

  • Talc's luster can range from pearly to greasy to dull.  “Talc, Argonaut Quarry, Ludlow, Windsor County, Vermont 2” by Stephanie Clifford is licensed under CC By 2.0
    What is the concept of luster in gemology?

    The term luster is used to describe the way a mineral surface looks when light is reflecting off of it. Luster is considered a basic descriptive parameter for minerals but varies somewhat even within a single crystal, which limits the term’s…

  • optic19
    The Optical Properties of Gemstones 

    Step 2: Advanced Gemology Lesson 1 Back to syllabus The Properties of Light Light is is such a common thing we tend to take it for granted. We have been experiencing it since birth and are very familiar with it; however, rarely do…

  • Andalousite_taillée
    What Is Gemstone Pleochroism?

    Gems that show two or three colors when viewed from different angles are said to be pleochroic (showing “more colors”). This quality is called pleochroism and can be found quite spectacularly in gemstones such as cordierites, andalusites, and…

  • Trapiche Emerald

    What is a Trapiche Emerald? There are plenty of books and articles describing the beauty of the emerald, from its rich green tones to the jardin, (garden) inclusions that often occupy an emerald (and help to identify its origin) to its unique…

  • gemstone color change - stillwellite
    Gemstone Color Change Phenomena 

    Frequently, people ask me to explain how gemstone color change works. Some stones can change color under different light sources. Although the answer is pretty complex, a few diagrams can really help.

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