Minerals & Gemstone 480x104
Minerals & Gemstone 480x104
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MINERAL PROPERTIES: OPTICAL PROPERTIES


Several important optical properties are applicable to minerals and gemstones, and can be very useful for gem identification. With proper equipment, jewelers can easily distinguish a Ruby from Garnet or red glass, even if their outward appearance may be identical.

White Light, or visible light, is a form of electromagnetic radiation (energy waves produced by the motion of an electric charge). White light belongs to the color spectrum, which defines all forms of light and electromagnetic radiation. The spectrum also includes many forms of light not visible to the human eye, such as ultraviolet and infrared light.

The rate of motion produced by the electrical charge defines the wavelength of light. Different wavelengths produce different types of light; white light encompasses all wavelengths visible to the human eye. White light contains the seven primary colors: red, orange, yellow, green, blue, indigo, and violet.

The color white is a composite of all colors. Every substance receives its color from the way white light reacts to it. Light can either be absorbed into a substance, or it can be reflected. The presence of certain elements or chemicals in a substance determine which wavelengths (i.e. colors) are reflected and which are absorbed. The wavelengths reflected off a substance determine its color. For example, if a substance absorbs all wavelengths except for yellow and green wavelengths, which are reflected, the color of the substance is yellow-green. If a substance absorbs no wavelengths but reflects them all, its color is white. If, however, a substance absorbs all wavelengths, its color is black.

In addition to reflection and absorption, light can also be passed through a substance. Light passing through a substance determines its transparency. If all light passes through a substance, and none is reflected or absorbed, the substance is transparent and colorless. These three attributes (reflection, absorption, and passing through) determine the color and transparency of a substance.


Optical properties relevant to minerals and gems:

  • See also "Color" in mineral properties


REFRACTIVE INDEX (RI)  

The speed of light varies in substances. The speed of light is different in air, water, and other dimensions, including minerals and gemstones. When light travels from one dimension to another dimension, the light bends, or refracts, upon entering the second dimension. This phenomenon can be witnessed with a stick protruding from a pond, where the stick appears to "bend" at the water level. This is caused by the difference in the speed of light in air and the speed of light in water. How much the light will bend, or the angle of refraction, depends on difference in the speed of light of the two substances. All transparent gemstones refract light, since the speed of light is different in the air than in gemstones.

Every gemstone refracts at a distinct, individual angle. The angle of refraction is directly related the speed of light in the gemstone. The refractive index of a gemstone measures the difference between the speed of light in air and the speed of light in the gemstone. This is determined by the gemstone’s angle of refraction. Every gemstone has a unique refractive index, meaning every gemstone refracts light at a unique angle.

The refractive index value measures how much slower light travels in the gemstone than in the air. For example, the refractive index of Diamond is 2.42. This means that the speed of light in Diamond is 2.42 times slower than the speed of light in air.

Refractive indices of minerals range from 1.2 to about 3. However, gemstones with a refractive index greater than Diamond (2.42) are either synthetic or are too soft for practical gemstone use. The greater the refractive index of a gemstone, the more brilliant or lustrous it is.

The refractive index of a gemstone is measured with a refractometer, a tool that measures angle of refraction. This tool is used by almost all gemologists and professional jewelers, for it provides simple, inexpensive, and accurate gem identification. However, a refractometer cannot read values greater than 1.86. Gemstones with refractive indices greater than 1.86 can only be tested by placing them in a liquid with a known refractive index, and then calculating the difference in refraction between the liquid and the gem.


DOUBLE REFRACTION (DR) 

Another optical property, known as double refraction or birefringence, is present in all non-amorphous minerals that do not crystallize in the isometric crystal system. When light rays enter birefringent minerals (minerals with double refraction), the light divides into two rays. The two rays differ in their angle of refraction. Therefore, all birefringent minerals have two refractive indices, one for each ray. The double refraction in most minerals is so weak that it cannot be observed with the naked eye. However, a small number of minerals have a strong double refraction, which is easily seen when the crystal placed over an image appears to "double" the image.

Double refraction is an important guide to gem identification. When viewed through a refractometer, birefringent minerals show two readings – one for each refracted ray of light. Double refraction is a characteristic trait, meaning every specimen of the same gem always has the same double refraction.

Double refraction is measured by the difference of refraction in each light ray. For example, if a gem is placed in a refractometer and shows a double reading of 1.62 and 1.63, its double refraction is .01.

Highly birefringent gemstones, such as Zircon, must be cut in a way where the double refraction is least noticeable in the finished gem, for it otherwise appears blurry.


DISPERSION & FIRE  

Dispersion is the splitting of white light into the colors of the spectrum. This effect is observable in faceted, transparent, colorless gems, where the white light disperses in the gem and reflects on its inner surfaces, giving the gem a colorful sparkle. This effect is known as fire in the gem trade. Generally, gems with higher refractive indices display greater fire. Diamond has the greatest fire of all true, non-synthetic gems. Room lighting conditions play an important role in fire, for the stronger the light, the more intense the fire appears to be.

The design of the brilliant cut was extensively researched to offer a gemstone its maximum amount of fire. For this reason, many transparent gems with high dispersion are faceted with the brilliant cut.


ABSORPTION SPECTRUM  

The absorption spectrum describes the spectral wavelengths absorbed by a gem. The chemical structure of each gem allows only certain wavelengths to be absorbed (the rest are reflected or pass through). The wavelengths absorbed into a gem can be detected with an instrument known as a spectroscope. Color alone cannot be used to identify a gem, for many gems have identical colors. A spectroscope examines the "true" color of a gem. Examining the absorption spectrum of a gem is one of the most useful and practical methods of gem identification.


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