by Erum Qureshi
Step by step laboratory identification procedure for Rubies. “There are numerous tests conducted to identify true rubies. Various characteristics and traits are judged by a gemologist to authenticate and grade a ruby”
Identification of Rubies
There are numerous tests conducted to identify true rubies. Various characteristics and traits are judged by a gemologist to authenticate and grade a ruby.
The first criteria a gemologist looks for in a stone purported to be a ruby is the color – a strong red color, and a stone belonging to the corundum species will be the first very basic factors, which will decide if the given stone is indeed a ruby and whether it ought to be subject to further examination and tests to prove beyond doubt that it is indeed a ruby. Tests for hardness, for measuring the specific gravity of the stone and the refraction of light are just some of them.
Physical Properties of Rubies
Corundum as a mineral would be colorless if pure. However the presence of one or more metallic oxides present in the stone as impurities gives corundum its colors. All stones belonging to this species ranging in varying shades of red to a deep crimson are called rubies.
Light red, pink or even violet corundum are usually called sapphires because if they were grouped as rubies they would be stones of an inferior quality. Hence, besides red, all other colors of corundum are properly termed as sapphires with color prefix.
Corundum is found in many colors besides red (ruby) and blue (sapphire). Colorless, yellow, violet, green and even brown colors may be found. The name ruby literally means red (derived from the Latin word – ‘ruber’.) Rubies vary in shades from varieties of red to a deep crimson due to traces of chromic oxide (Cr2O3) in its crystal structure. The amount of chromic oxide (usually 1-3%) determines the depth of color in the stones and traces of iron in the ferric state gives them a brownish tinge. The ideal color for a ruby would be a strong traffic signal red.
Although corundum is a hard mineral (rating 9 on the Mohs scale of hardness) care needs to be taken during testing as they are slightly brittle and if dropped on a hard surface or given a sharp blow, are prone to develop internal flaws or fractures.
Step by step tests to determine Ruby’s authenticity
Determination of Specific Gravity (SG)
Each gemstone has its own specific gravity which is ratio of its weight to the same volume of water.
The specific gravity of purest corundum is around 3.98, this value, way beyond the range of commercially sold heavy liquids usually used to determine Specific gravity. Therefore a special solution called “Clerici” is used. This solution contains very soluble thallium salts: the ‘formate’ and the ‘malonate’. The Clerici solution at a specific gravity of 4.00 is dense enough to float corundum at room temperature. Thus, in spite of being hazardous and viscous as compared to other high density liquids, it is used to measure the specific gravity of Ruby and Sapphire.
The Clerici solution is an aqueous solution and can be diluted with distilled water. Ruby when suspended in a beaker of Clerici solution has to be clean and dry and so do the tweezers and other apparatus used for the test. Care needs to taken that these are free from other organic solutions before immersing the gem in the solution.
Natural ruby will have an SG approximating 3.99 although there is a variation for stones mined from different localities. Once the SG is confirmed to be that of corundum, the gemologist moves on to the next step.
Determination of Refractive Index
Light rays leaving one medium and entering another obliquely seem to bend a little at the place of contact. This is called refraction of light.
During gem testing, measuring the refractive index or RI helps the gemologist observe the indices of refraction of light on a particular species of gemstone. Spot readings are taken for cabochon cut rubies; faceted stones are placed facedown on the glass of a Refractometer with a small amount of contact liquid and the reading is taken. The RI of ruby will be between 1.76 and 1.77.
The refractive index can sometimes be a decisive factor for successfully eliminating a ruby look-alike like tourmaline which will have a much lower RI of 1.61 to 1.66.
Double Refraction Test
When a ray of light enters a crystal having a Trigonal system in directions other than the optic axis, it splits into two distinct rays. This is known as double refraction. The Ruby, being of the Trigonal crystal structure shows two indices of refraction in a polariscope. These are known as the ordinary ray and the extraordinary ray.
This can be useful in identifying gemstones, the difference between the highest and lowest refractive index is known as birefringence. Ruby has a birefringence of – 0.008, with a negative sign because the ordinary ray is of greater value.
Because the ruby is doubly refractive (it has two rays), each of these rays absorbs light differently and this has an effect on the stone’s color. When it is turned, it shows a slightly different shade of color and this effect is known as Dichroism.
The ordinary ray in ruby showing a deep purplish red is the most attractive and desirable for this species.
Determination of Absorption Spectra
The absorption of colored stones is in well marked bands or finer dark lines which cross the otherwise continuous spectrum, obliterating certain colors or wavelengths when white light is transmitted through or reflected from the stone.
The spectroscope is an instrument that determines the wavelength of the absorbed light. The wavelength is measured in nanometers (symbol nm) or angstrom symbol ?. The spectrum most common to Ruby is the chromium spectrum. It is characterized by general absorption in the deep violet, transmission in the blue, an absorption band in the green and transmission in the orange and red.
The luminescence of rubies whose coloring agent is chromium is a strong crimson / orange light when subject to any kind of radiation (short-wave, long-wave or even blue light). When viewed through the Chelsea color filter, this fluorescent red is visible. It is this property that can be used to distinguish between garnets and rubies – garnets do not fluoresce.
Without doubt the final and most determinative test to decide the authenticity of any gemstone is its examination under a microscope.
Gemologists rely on their expertise and experience to spot the inclusions before taking the final call on the authenticity and grade of a gemstone. Inclusions (irregularities in the crystal structure and internal flaws) in ruby are straight angular growth lines following the crystal faces. Included are rutile needles or hematite plates called ‘silk’, which bring about a soft sheen in the stone.
When such a stone is cut en cabochon these rutile needles or ‘silk’ brings about the rare cat’s eye effect or the very desirable asterism (a six rayed star that shimmers above the stone’s surface when it is moved). Rhombohedral twinning and boehmite needles are inclusions exclusive to natural corundum and have never been observed in flux grown synthetic corundums. Inclusions typical to synthetic ruby are curved growth lines (thin striae or bands), gas bubbles and dense clouds of extremely fine rutile silk in star stones. These clouds do not show the straight angular zoning patterns that natural stones show, they have curving bands instead.
Besides the observing the color, there are an array of tests to identify a given stone as a ruby. These tests, conducted by trained gemologists under strict guidelines using highly sophisticated equipment will establish beyond doubt the identity of any given gemstone. As it is next to impossible for an untrained eye to distinguish between synthetic and natural stones, or even ruby or a look alike from a gem of another species, these methods are valuable tools that help in identifying gemstones and making a correct, informed decision about their purchase or use.