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Dichroscope

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DichroscopeA Dichroscope is a gemological instrument used by gemologists on transparent colored stones to aid in identification. The most common type is a calcite-type, but polarizing types are used as well. They are usually tubular in shape and approximately 2 inches long and 1/2 inches in diameter with a round opening on one end used as a view piece, and a rectangular opening on the other.

Transparent minerals can be divided in to two categories. As a ray of light enters a material, it will either pass thru as a single ray of light (single refraction) or two or more rays (double refraction). By observing a material thru the dichroscope and understanding the characteristics of various materials, one can make certain assumptions about what the material is. For example, diamonds, garnets, spinels, and glass are all single refracting. On the other hand, gemstones such as sapphires, rubies, and emeralds are double refracting.

When observing a double refracting material thru the dichroscope, different colors will appear in two squares at the opposite end. Since each mineral will have a unique pairing of colors, you can use these color pairs to differentiate between true emeralds and emerald-like stones, such as tourmaline and zircon.

For green color materials, some common color pairs that will be observed thru a Dichroscope are shown below.

Material
Intensity
Color A
Color B
Color C
Andalusite
strong
brownish-green
olive-green
flesh-red
Emerald
varies
yellow-green
bluish-green
Alexandrite
strong
emerald green
yellowish
reddish
Green Sapphire
strong
yellowish-green
green
Green Hiddenite
strong
bluish-green
grass-green
yellow-green or colorless
Peridot
moderate
yellow-green
green
yellowish
Green Tourmaline
strong
pale-green or brownish-green
strong-green or dark-green
Zircon
weak
brownish-green
green or colorless


When properly used, the user will observe the stone from five different directions, top to bottom, diagonally thru the facets on the crown once each from opposing sides, from the side perpendicular to the girdle, and again perpendicular to the girdle 90 degrees from the previous. By doing so, you are insured that you are observing the stone in its entirety, reducing the risk of being fooled by a doublet? or triplet.

By comparing the colors seen in the two square boxes against the chart above, one can make reasonable assumptions as to the type of mineral being analyzed. It is important to note however, that a dichroscope cannot differentiate between synthetic and natural materials; for this, other tests must be used. However, when properly used in conjunction with other tools such as a Jewelers Loupe and Chelsea Filter, certain assumptions about a gemstones composition can be made with a very high degree of accuracy.

Photo © 2007 by Embassy Emeralds, All rights reserved.

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