These include the loupe, tweezers, dichroscope, ultraviolet light torches, and polariscope. Part III continues the series with tools that can make emeralds appear red, drag gemstones across tables, and calculate relative density.
Chelsea Colour Filter
Although the Chelsea Colour Filter (CCF) is a very simple instrument in the gemmologist’s arsenal, to understand the tool you must understand the nature of light.
White light can be broken up into the same seven spectral colours seen in a rainbow – red, orange, yellow, green, blue, indigo, and violet, each with a slightly different wavelength. This breaking up of light is called dispersion, and is what is happening when you observe 'fire' in a diamond.
The CCF features a sheet of transparent green optical plastic that allows only the deep red and a certain narrow band of yellow and green wavelengths of light to pass through it, while all other wavelengths (colours) are absorbed by it.
The CCF was originally designed to distinguish emeralds from imitants like green paste, which is glass.
In this example, the CCF blocks the green wavelengths normally seen passing through emerald and allows the red wavelength also transmitted in some gemstones, such as those from Colombia, to be seen.
Compare this red reaction for some emeralds with the yellow/green reaction observed in the paste imitation and these two stones can be successfully separated.
Specific gravity
Gemstone species have varying densities owing to the elements present within them and their arrangement, some being heavier such as iron, and others lighter such as silicon.
Gauging the relative density of a gemstone can be very useful in identifying the mineral family the gemstone belongs to.
This is determined in gemmology by calculating specific gravity, also known as relative density - the ratio of the density of the gemstone compared with water, a known density at 4°C.
This idea of determining relative density began with the principle of buoyancy, proven by and subsequently named Archimedes principle after the renowned mathematician of Ancient Greece.
Tasked with determining whether the king’s crown was pure gold, he needed a method of calculating density. The story is famously recounted as Archimedes taking a bath, where he realised that an object immersed in a liquid experiences a force equal to the weight of the volume of liquid it displaces.
With further experiments comparing equal weights of pure gold and pure silver immersed in water, he was able to determine the density of the crown and concluded it was alloyed with silver.
Building upon this discovery, another acclaimed mathematician introduced the method by which we measure specific gravity today – Galileo. Using two-pan scales to measure objects in air and in water, he formulated the equation gemmologists use to calculate the specific gravity of a gemstone.
The apparatus for this test can either be rudimentary and portable, or highly precise and calibrated. Either way, the gemstone in question is weighed on the scales provided in air, followed by in water.
The calculation itself is simple – the weight of the gemstone in air is divided by the loss of weight when weighed in water.
As one of only two gemmological instruments that provide a result as a numerical value, the outcome can then be assessed against a book of constants that give the range of specific gravities of different gemstone species.
Neodymium magnets
Some gemstones contain elements or metallic inclusions that make them magnetic.
Examples in the former category include almandine garnets, due to their iron content, while an example in the latter includes some HPHT laboratory-created diamonds.
Some gemstones may have a weak reaction to a strong magnet, whilst others may jump from the table to a magnet. This property makes neodymium magnets, the most powerful permanent magnet, another insightful yet compact tool at a gemmologist’s disposal.
In the next issue of Jeweller, the fourth and final instalment in the Tools of the Trade series, we will explore how a microscope can act as a window to a gemstone’s origin.
We will also examine how the bending of light can be measured by the refractometer and what a rainbow can reveal about chemistry in the spectroscope.
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