Gemological Properties of Gemstones

Gemstones have certain properties that gemologists rely on to aid in identification. These include density and refractive index and visual characteristics among others.


Density refers to the physical property of matter that describes how tightly packed together the atoms of an element or molecule of a compound are. The more tightly packed the individual particles of a substance are, the denser the substance is. Understanding density is a useful tool in identifying minerals since substances have different densities. For example, a ruby and an emerald of the same size and cutting style will have vastly different weights. This is because the density of the ruby is greater than that of the emerald. Therefore, a one carat ruby will be smaller in size than a one carat emerald.

In gemology we measure density via the specific gravity (SG) of a gem. Specific gravity is defined as the ratio of the weight of a substance to the weight of an equal volume of water. The SG of a gem can be measured using a scale and water. Specific gravity varies with chemical composition and crystal structure type. Therefore it can be very useful in identifying minerals. More commonly, however, the SG of gems is approximated by comparison using heavy liquids. Gemologists use sets of liquids of known SG values to perform the comparison. The stone (faceted or rough) of unknown identity is immersed in the heavy liquid of know SG. The stone is observed to see if it floats (lighter than), sinks (heavier than) or is suspended in (equal to the weight of the liquid). Depending on the result, the stone may be cleaned and then immersed into a different liquid within the set. This process is repeated until the SG of the stone is obtained or all other potential gem varieties have been excluded by process of elimination. Once the SG is obtained, the gemologist will refer to a gemological properties chart of SG values to match it with the corresponding gemstone.

Refractive Index

Another important property used to identify gems is the refractive index (RI). After visual observation, the RI is often the first test a gemologist will perform to identify an unknown gemstone. Refraction occurs when light enters a transparent or translucent material or liquid from air causing its speed and direction to change. We have all observed this phenomenon in life. For example, the underwater portion of a canoe paddle appears at an angle to the handle portion above the waterline. As light enters a gemstone its speed is reduced. This sudden reduction of speed causes the light to bend. Therefore, refractive index refers to how many times faster light travels through air than through an object.

The refractive index of diamond is 2.42. Essentially this means that light travels through air at a rate of 2.42 times faster than it travels through diamond. The slower light moves through a gem the higher the RI will be. The instrument used to measure RI is called a refractometer. Once the RI of the subject stone has been determined the gemologist will refer to a chart of known RI properties to identify the stone.

Because gems of the same variety can have slightly different chemical compositions or structures, there can be slight differences of RI value, so RI is often expressed as a range on gemological data charts. For example, the RI of emerald can range from 1.577 to 1.583. However, each gem type has a distinct RI. Although there can be overlap between related materials, visual observations are sufficient to make the proper identification. For example, the RI of garnet and ruby can overlap. However, the single refractive nature of garnet (light travels at the same speed regardless of direction) is sufficient to separate it from doubly refractive gems (light splits into two beams each traveling at a different rate of speed) such as ruby.

Gemologists use a myriad of other characteristics and properties to identify gem materials. These can include the hardness of the stone, observation of diagnostic inclusions, growth zones and color. The gemologist’s best asset is experience.