Gemstones that have the optic characteristic known as “doubly refractive” split light as it passes through the gemstone. Each resulting beam of light has its own Refractive Index. The difference (range) between the two refractive index values is known as its “birefringence” value.
The amount of “dispersion” in a gemstone is meant to provide a relative comparison as to how much spectral colors are visible under certain conditions. The presence of visible spectral colors in the gemstone is commonly known as the “fire” of the gemstone. Dispersion is a scientific comparison between the Refractive Indexes of two different wavelengths of light passing through the same material. How much “fire” the gemstone has is based on many different factors, including the refractive index of the material, the gemstone’s facet angles, the quality of the gemstone’s polish, the lighting used in the test and other factors. Gemstones with higher dispersion values are known to have more “fire”.
How difficult it is to scratch a material is known as the material’s “Hardness”. Gemstone hardness is compared to the Mohs Scale. This scale was defined by German mineralogist Friedrich Mohs in 1812 by comparing 10 commonly available minerals. Ranking of the materials was performed based on the ability of a mineral to scratch (or not scratch) another mineral. On the Mohs Scale, a higher number represents a harder material. A lower number represents a less hard material. Given the nature of the scale (based on scratch observations, not scientific measurements), the scale is not mathematically linear, but arbitrary. For example, sapphires and rubies are considered to have a hardness of 9 on the Mohs Scale, but are considered twice as hard has their predecessor, topaz, which has a hardness of 8. On the Mohs Scale, diamond has a hardness of 10, but is four times harder than those materials with a hardness of 9.
The manner in which light interacts with the surface of a gemstone is known as its “Luster”. This property of a gemstone is more of an observation used for comparative purposes than it is a scientific means of comparing. Several different factors play into the luster grading of a gemstone, which in its simplest form, just describes how light reflects on the gemstone surface. How well light reflects on the surface of the gemstone is dependent on other properties of the gemstones, such as its Refractive Index, its Hardness, and its level of polish.
When a beam of light enters a gemstone, the structure of the gemstone will determine if the light will continue passing in a single beam, or whether the beam will split. Gemstones where the beam of light does not split are called “singly refractive” or “SR”. Those gemstones that split light beams are known as “doubly refractive” or “DR”. Doubly refractive gemstones have two Refractive Indexes; one for each split beam of light. When a gemstone is doubly refractive, the difference in the refractive indexes of the gemstone is called the Birefringence.
As light passes through an object, it has a tendency to bend as the speed of the light slows down. The amount of “bend” that a material yields when light passes through it is known as “refraction”. When light passes through a vacuum (nothing), it bends very little. This slight bending of light is used to compare how much a given gemstone bends light passing through it. This ratio is known as the gemstone’s “Refractive Index”. Gemstones with a higher refractive index are more brilliant than gemstones with lower refractive indexes. Most gemstones have refractive indexes noted as ranges instead of a single number. This is due to the Optic Character of the gemstone.
The weight of a material is a result of its density; how much mass it has in a defined space (volume). Those materials that are denser weigh more than other materials that occupy the same volume. “Specific Gravity” is a comparison of a material’s weight compared to the weight of the same volume of water. When comparing gemstones, those with higher specific gravity values will weigh more per volume than those with lower specific gravities. So a 2mm gemstone of one material that has a specific gravity of 4.3 will weigh more than an equally-sized 2mm gemstone with a specific gravity of 3.5.
How resistant a material is to chipping and breaking is considered the material’s “Toughness”. Chipping and breaking are the result of stress, or force, being applied to the material. Gemstones are typically compared based on their ability to absorb force, and are described as Exceptional, Excellent, Good, Fair, or Poor. Gemstones that can take a beating and not break are “Exceptional” and those that fail quickly are considered “Poor.”