Know Your Aquamarine
With a name that literally means sea water, the most acceptable color for this stone is a clear transparent sky-blue. Much Aquamarine has a bluish-green color, and the color is caused due to the presence of iron unlike emerald in which chromium is the coloring agent.
Placing a spotlight on the immensely popular member of the Beryl family- Aquamarine we delve into the beautiful depths of this gemstone. With a name that literally means sea water, the most acceptable color for this stone is a clear transparent sky-blue. Much Aquamarine has a bluish-green color, and the color is caused due to the presence of iron unlike emerald in which chromium is the coloring agent.
Other varieties of the mineral beryl are the hugely popular emerald, the pink to peach-pink variety morganite named after the American banker JP Morgan who is also an enthusiastic gemstone collector and the colorless goshenite. There is also a yellow variety of beryl known as ‘heliodor’ (a name derived from the Greek words meaning sun and gift) that occurs in a wonderfully rich golden-yellow color.
Many Brazilian states provide large quantities of good quality beryls including aquamarine and other colored beryls, the most famous amongst them being Minas Gerais. Small, deep blue aquamarine is also found in localities in the African nations of Kenya, Nigeria, Zimbabwe and Zambia. The Ural mountains, Madagascar, scattered localities in the US, Namibia and Burma (Myanmar) are also some other places where gem quality aquamarine is mined.
Aquamarine is found in pegmatites as well as water-worn pebbles. Unlike emeralds the stones are found in large crystals of flawless clarity from which large clear stones can be cut. Cut aquamarines need to be of some considerable size for the color to be sufficiently intense to produce a good colored stone.
As mentioned above aquamarines need to be of a considerably large size for the color to be sufficiently intense, most blue aquamarine encountered in the market today is the result of heat treatment on greenish-yellow or brownish-yellow stones. The blue color is induced by heating to a temperature between 250 and 720 degree Celsius for varying time periods. The resultant color is permanent.
Aquamarine like emerald has a tendency to brittleness, therefore care should be taken while handling the stone and setting it in jewelry.
Aquamarine exhibits distinct Pleochroism. When viewed through a dichroscope ‘twin colors’ can be observed. These colors are deep blue and colorless and their strength depends on the depth of color of the stone. Aquamarine can be easily picked out of a parcel of similar looking stones by viewing them together through a Chelsea Color filter. The aquamarine will appear a strong greenish-blue under the filter.
The most convincing imitation is offered by synthetic spinel colored pale blue by cobalt. However these stones have an entirely different refractive index (1.72) and specific gravity (3.63) from those of aquamarine (RI: 1.57 to 1.58, SG: 2.65 to 2.8). The quickest and easiest test for a busy jeweler in this case is to place the suspected stone close to a strong light and view it through a Chelsea color filter held close to the eye. The synthetic spinel will show a distinct red under it whereas an aquamarine would show green.
Imitations in pale blue glass are sometimes deceptive to the eye but are easily distinguished by the characteristic inclusions of glass- gas bubbles, swirls and mold marks. Also on a Refractometer a glass gem will show a single shadow edge as glass is isotropic.
The only natural stone closely resembling aquamarine is blue topaz, which gives Refractometer readings of 1.610 and 1.620 as compared to the 1.57 and 1.58 which are the more common readings for aquamarine.
Although generally inclusion free and eye-clean, aquamarine has characteristic inclusions that appear as hollow tubes- known as rain.
Chelsea Color Filter (CCF)
The Chelsea Color Filter was originally developed in the laboratory of the London Chamber of Commerce in 1934 and subsequently marketed by the Gemological Association. Through this filter a typical emerald of good commercial quality will appear red. All green pastes, most soude emeralds, green sapphire and most tourmalines will show no red through the filter.
Blue gems in which cobalt is the coloring agent also appear red through the CCF. Hence synthetic blue spinel, blue cobalt glass, or a doublet with a blue cobalt glass base all appear red or an orange-brown color (in case of the spinel). A word of caution though; natural blue spinel can appear reddish through the filter and so can a Sri Lankan sapphire containing a touch of chromium as the coloring agent.
Some Common Reactions Under Chelsea Color Filter
|1.||Synthetic Emerald (Cr)||Red|
|2.||Blue Glass (Co)||Red|
|3.||Synthetic Blue Spinel (Co)||Red|
|4.||Synthetic Blue Quartz (Co)||Pink|
|5.||Natural Aquamarine (Fe)||Green|
|6.||Natural Zircon (Blue)||Green|
|7.||Dyed Green Chalcedony||Red to Orange Red|
|8.||Dyed Blue Chalcedony||Red to Pink|
|9.||Natural Emerald (Cr/V/Fe)||Pink to Red / Green to Yellow Green|
|11.||Blue Topaz||Blue Green|
One test to determine a gem’s identity is to measure the refraction of light in the gem. Every material has a critical angle, at which point light is reflected back internally. This can be measured using a Refractometer and thus a gem’s identity could be ascertained.
A gemological Refractometer is a device that projects the rays of light reflected from a gemstone onto a scale. This scale can be seen through the eyepiece of the instrument to be partly in shadow and partly brightly illuminated. The position of the shadow edge enables one to read the refractive index of the gemstone and also measure its birefringence.