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Learning to look closer
Gemology is the study of gemstones, including natural and laboratory-created materials. Gemologists use observation and systematic testing to identify a stone and describe its properties. Understanding durability, care, and any treatments helps you choose a gem suited to how you plan to wear or enjoy it.
Hardness
The Mohs scale compares resistance to scratching from 1 to 10. A higher-ranked mineral can generally scratch a lower-ranked mineral. The numbers are relative, not evenly spaced: diamond at 10 is much harder than corundum at 9. Hardness is only one part of durability; a hard gem may still chip or break.
Everyday reference points: a fingernail is about 2.5, a copper coin about 3, ordinary glass around 5–6, and a steel nail around 5–6. Exact values vary.
Refractive index
Light changes speed and direction as it enters a gem. Refractive index describes how strongly the material slows light relative to its speed in a vacuum. Measured RI can help distinguish gemstones, though identification usually considers several properties together.
Specific gravity
Specific gravity compares a material’s density with that of water. Two gems of the same dimensions may weigh different amounts because their densities differ. It is a useful identification clue, especially for loose stones.
Chemical composition
A mineral’s chemical formula describes the elements in its structure. Chrysoberyl, for example, has the formula BeAl₂O₄: beryllium, aluminum, and oxygen in a 1:2:4 ratio. Composition is one part of a mineral’s identity.
What else can we observe?
- Color
- Hue, tone, and saturation; appearances may change under different lighting.
- Transparency
- How much light passes through a material.
- Luster
- How light reflects from the surface.
- Cleavage & fracture
- How a material breaks along structural planes or across them.
- Dispersion
- White light separating into spectral colors, sometimes seen as “fire.”
- Fluorescence
- Visible light emitted while a material is exposed to ultraviolet light.
- Pleochroism
- Different colors or shades seen from different viewing directions in some gems.
- Optical phenomena
- Distinct effects such as asterism, chatoyancy, or color change.
A PRACTICAL BUYER’S GUIDE
What to look for in a faceted gem: The Four Cs
Color, clarity, cut, and carat weight are useful starting points, but they do not carry equal weight for every gem. Compare stones of the same variety, ask about treatments, and look at the actual stone in more than one kind of light whenever possible.
Color
Look at hue, how light or dark the stone appears, and the strength of its color. Turn it to check whether color is even, patchy, or lost in dark areas. What counts as desirable depends on the gem variety and your own taste.
Clarity
Inclusions are natural features inside many gems. Ask whether they are visible without magnification, where they sit, and whether a fracture could affect durability. A clean aquamarine and an included emerald should not be judged by identical expectations.
Cut
Shape is the outline; cut quality is how well the facets, proportions, symmetry, and polish work together. View the gem face-up and tilt it gently. Notice bright areas, lifeless dark areas, or a pale window through which you can see straight through the center.
Carat weight
A carat is a unit of weight equal to 0.2 grams, not a measurement of diameter. Gems of different densities can have different face-up sizes at the same carat weight. Check millimeter dimensions as well as the carat figure.
Why clarity grades need context
Some transparent gem varieties are commonly eye-clean (often called Type I); others commonly show inclusions (Type II), while varieties such as emerald commonly have visible inclusions (Type III). These are expectations for a variety, not a ranking of beauty or value across different kinds of gems. Even different colors of tourmaline can fall into different clarity types.
You may see codes such as VVS, VS, SI1/SI2, and I1/I2/I3. They describe progressively more noticeable inclusions, but the appearance associated with a code depends on the gem’s clarity type. “Eye-clean” means no inclusions are apparent to the unaided eye at normal viewing distance. A grade alone cannot tell you whether an inclusion is attractive, distracting, or a durability concern.
Questions worth asking before you buy
- What is the gem’s identification, and is it natural, laboratory-created, or an imitation?
- Has it been heated, filled, dyed, irradiated, or otherwise treated? Is the treatment stable, and does it change care requirements?
- Can I see clear photos or video in neutral lighting from more than one angle, including the back?
- Are inclusions or surface-reaching fractures visible in person? Do they affect durability?
- What are the dimensions in millimeters, the carat weight, and the return terms?
- For a consequential purchase, is an independent report available for this individual stone, and what exactly does the report establish?
Further reading: Blue Nile on colored gem clarity · The Natural Gemstone Company on faceted gems · International Gem Society on clarity codes and types.
