Lab identification of Sapphire
by Erum Qureshi
Sapphire, derived from the Greek word sapphirus, literally means blue. Many people will find it difficult to believe that ruby, with its intense red color and the royal blue sapphire are the same mineral corundum composed of oxygen and the light metal aluminium.
Ideal sapphires are stones of an intense blue verging on the violet. Large sapphires are more common than large rubies. Let us look at the physical properties of the mineral species corundum to which the sapphire belongs.
Metallic oxides are responsible for imparting color to this mineral. If not for these impurities present in the crystal structure of corundum, it would be colorless. Red stones of the corundum species are termed as rubies and all other colors are known as sapphires with the color prefix.
Corundum is found in many colors besides red and blue. Colorless Sapphire is called leuko-sapphire, yellow, violet, green and even brown colors may be found. Also sometimes found is a very rare and highly valued orange color, known as ‘padparadscha’ (from the Sinhalese word for ‘lotus flower’). Occasionally color-changing sapphires can be found; asterism and star-stones come in almost all colors.
Hardness is the same as ruby (9 on the Moh’s scale) and also differs differently in different directions (an important factor which need to be kept in mind during cutting).
Determination of Specific Gravity
Specific gravity of a gemstone is the ratio of its weight to the same volume of water. Since there is no definite demarcation between ruby and sapphire, the same tests for measuring specific gravity are used as those for ruby. The Clerici solution is used to measure the specific gravity of sapphire as this solution is the heaviest among the high-density liquids available. The stone when suspended in the Clerici solution should be clean and dry as the tweezers and other apparatus used for the test. Care is should also be taken that these are free from other organic solutions before immersing the gem in the solution. Natural sapphire will have an SG approximating 3.99. An exception is iron-rich yellow, green and blue-green sapphires, which may have specific gravity as high as 4.00. Once the SG is confirmed to be that of corundum, the gemologist moves on to the next step.
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. The refractive index for sapphire is measured exactly in the same way as that of ruby, both being of the same species, their RI is the same. Spot readings are taken for cabochon cut sapphires; 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 sapphire will be 1.76 to 1.77.
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 Sapphire, 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. Birefringence is the difference between the highest and lowest refractive indices. Sapphire has a birefringence of – 0.008, with a negative sign because the ordinary ray is f greater value.
The fact that there are two rays in these stones has an effect on color, for each of these rays absorbs light differently and as the stone is turned it shows a slightly different shade of color. This effect known as Dichroism, is seen fairly well in sapphires of all colors except in colorless and yellow stones. The most attractive color is that of the ordinary ray in sapphire, which is a deep royal blue.
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 blue, green and yellow Sapphire is the iron spectrum. It consists of three lines of increasing intensity at 451, 460 and 470 nm. The absorption spectra of sapphire (in nanometers):
Sapphire ,blue (from Australia) : 471, 460, 450.
Sapphire, blue (from Sri Lanka) : (450)
Note: Strong absorption lines are in bold letters; weak ones are in parenthesis.
In the case of blue sapphires, UV fluorescence is practically non-existent. Except maybe Sri Lankan sapphires, which have chromium traces. These show a red or orange glow under long wave and weak blue glow under short wave.
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. The best stones are those with no major color zoning problems, which are well-cut, eye clean and most importantly of an intense blue color. Inclusions (irregularities in the crystal structure and internal flaws) in sapphire are straight angular growth lines following the crystal faces.
Included rutile needles or hematite plates are 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 stones 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 sapphire 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 but have curving bands instead.
Treatment and look-alikes
Through heat treatment at temperatures of about 1700-1800 degrees C, some cloudy sapphires, non-distinct in color, can change to a bright blue permanent color. Sapphires can be confused with benitoite, indicolite, iolite, spinel, tanzanite, topaz, zircon and even blue glass. Some imitations are made using doublets, blue cobalt glass with a crown of garnet or a crown of green sapphire and a pavilion of synthetic blue sapphire.
Today, synthetic sapphires having properties identical to the natural stone are being produced commercially.