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As With any other test, practice and perseverance are needed, and when the beginner has become accustomed to getting results in easy specimens he should strive to develop his skill to the limit and learn to gauge approximately the strength of the double refraction shown. the author has found this to be a most valuable accomplishment. the more thing that can be learned with a pocket lens the better-

even if one possessed other apparatus this will not usually be available in an auction room or on other people's premises. this same test can, of course, be extended and rendered easier b using a microscope, which functions as a much more powerful lens, magnifications of 25 to 60 diameters being quite comfortable usable with stone held between the thumb and forefinger of the left hand, supported on the microscope stage.

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If therefor in any two media which are in optical contact we can measure the angle where total reflection beings (i.e. the critical angle), and we known the refractive index of the denser medium. it is possible to calculate the refractive index of the rarer medium. this is the underlying principle of all total reflection refractometers (sometimes called total refractometers), but in the instruments designed for gem 

testing all calculation is ingeniously avoided. In such refractometer the optically dense medium of known refractive index is in the form of polished hemisphere of heavy lead glass, or a segment of such a hemisphere, or a truncated 60o prism of the same material, the flat upper surface in each case forming the 'table' of the instrument. if the flat, polished surface of any gemstones of lower refractive index than the hemisphere is place in optical contact with this table, rays

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Thus the ray AO is refracted along OA', apart from the small fraction of light which is reflected at O, as indicated by the dotted line. similarly, the ray BO is refracted along OB', as we consider rays which form greater angles with the normal, NOM, we reach an angle where the refracted ray only just grazes along the surface, OQ, between the two media. this angle is know as the critical angle, and all ray reaching O form the denser medium at angle greater than the critical angle are totally reflected back into the denser 

medium, it being physically impossible for any light form such rays to penetrate into the rarer medium. in the post, the angle COM can be considered as the critical angle of incidence; all the light s thus totally reflected along OC", and any other rays, such as DO, with a still larger of incidence, is, of course, also totally reflected. it is important to realise that the size of the critical angles will depend upon the relations between the reflactive indices of the denser and the rarer medium.

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It is the presence of traces of these in an otherwise colourless crystal that produces their colour varieties. more will be said about the function of these 'transition' element , when dealing with the spectroscope. granted that there are a considerable number of red, blue and green gemstones, there are very few that can 

approach ruby, sapphire and emerald in their rich shades of these colour and a colour sensitive and trained eye can go a long way towards distinguishing them from other natural gemstones, glass imitations, or even synthetic counterparts. it is indeed worthwhile for a beginning gemmologist to practice at every opportunity the sight identifications of the various gems and to pay special attention to the particular grades of colour typical of each species.

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Thus is must be realised that if, say, a zircon is cut with is table facet at right-angles to optic axis it may be necessary to tile the stone and make observations trough the bezel facets to see the double refraction well developed. also, as R. T. liddicoat has pointed out, in directions at right-angle to the optic axis of the uniaxial minerals, although the double refraction is at is greatest is cannot be directly observed, as the image due to the extraordinary ray is either directly in front or directly behind that the due to the ordinary ray. with zircon, 

sphene, and peridot the birefringence  is so strong that even the novice should have no difficultly in seeing the effect mentioned effect in quartz and corundum unless the stone are larger. (J.-P. Poirot, director of the paris gem-testing laboratory, has made the important observation that  for a given birefringence the doubling effects is more marked in stones of low R.I. than in those of high index.)

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While ruby and supphire are the red and blue varieties of the mineral corundum, there are also white, yellow, pink and green supphires. emerald and aquamarine are the rich green and pale blue varieties of beryls, but there are also white, pink, and yellow beryls, and green beryl that do not rank as emeralds, tourmaline, quartz, zircon and topaz are further well-known 

examples of such polychrome minerals, and tourmaline in particular is well known for frequently appearing as parti-coloured crystals, the boundaries between the colours -(usually green and pink) being quite sharp either across the length of the crystal or concentrically in zones. the reason of such confusing behaviour can usually be explained by the fact that there are a limited number (eight to be precise) of elements that give rise to colour in minerals.

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This deviation of rays of light on entering a new medium is called refraction. In figure 2.1 the rays of light, I.O, is refracted along OR  on entering the denser medium blow the surface PQ. The broken line show the path of  followed by the reflacted light. The extent of the bending of refraction of light on entering the stone depends upon 

its refracting power or 'refractive index', and this is inversely proportional to the velocity of light withing the substance. Put
differently, the refractive index of a medium may be define as the velocity of light in air* divided by the velocity of light in the medium. The velocity of light in air approximately I86,000 miles per second, and light from the sun and stars travel to us at this immense speed. In quartz (rock, crystal, amethyst, etc) the velocity is

reduced to approximately I20,000 and its diamond to only 76,860 miles per second. Thus diamond, in which light travels as mentioned above, at 76,860 miles per second in empty space, has a refractive index of I86,000\76,860=2.042; higher then the refractive index of any other gemstones used in jewellery-accounting for the brilliant, adamantine lustre of the stone.

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Colours plays an extermely important part in contributing to the beauty and popularity of precious stones. it was their colour that made such stones as turquoise and lapis lazuli among the first to attract the cupidity of early civilised man, and it is the mangnificent crimson red of ruby, the deep cornflower blue of the finest sapphires and the verdant green of emerald which (added to their transparency, hardness and rarity) have caused them 

always to rank amongst the supremely precious gems. only in the case of diamond, unique in this as in so many other ways, has a complete lack of colour been  regarded as the standard of perfection. those unversed in gemmology find it hard to realise that is most of the mineral species used as gems a wide colour range is possible, and that in their 'pure' state they would be colourless and thereby of relatively little value.

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When even the most practised eye may find it difficult to distinguish between stone of difficult species but of very similar colour, simple instruments can sometimes be used to 'analyes' the colour in one way or another and make the separation simple. what we call 'white' light, i.e. light from the sun or other incandescent bodies, is composed of a mixture of all the colours of the rainbow. newton, in the year of the great fire of london (1666), was the first to show that sunlight 

has a composite nature. he analysed a narrow beam of light that passed through a chink in the shutter into his darkened room, by the simple expedient of placing a glass prism in the path of the ray. light is variously refracted by a transparent solid according to its wavelenght, the red rays, of longer wavelength, beings less deviated than the shorter, violet, waves. When even the most practised eye may find it difficult to distinguish between stone of difficult species but of very similar colour, simple instruments can sometimes be used to 'analyes' the colour in one way or

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It has already been stated that each mineral has a definite refractive index by which it can be identified on the refractometer; list of these indices will be found on p. 27, and at the of the post. for the benefit of those who like to have at least a rough idea of the working of an 

instrument which they are using, a short description of the basic principle upon which all gem-tasting refractometer depend followers. however, those, who would like to know how to use a refractometer, but who prefer to shirk such explanation, can skip the next section and await the strictly practical directions which will be given later, on p.21. what followers can best be understood by considering the diagram, figure 2.2. here rays of light are considered passing from a dense medium into a rarer one, say from glass into air. the rays will then be refracted away

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As would appear in a singly refractive stone such as diamond, each edge appears as a double line (see figure 3.1). a word of warning, however, should be uttered here; according to their crystal structure, all doubly refracting stones have either one or two direction in which only single refraction obtains. those gemstones belonging to the tetragonal, trigonal or hexagonal system of crystals have only such direction; one 'optic axis' as it called. these minerals, which are grouped together optically us 'uniaxal', include zircon, 

corundum (ruby and sapphire), beryl (emerald and aquamarine), tourmaline, and quartz (amethyst and citrine). the minerals crystallising in either the orthorhombic, monoclinic or triclinic systems have two optic axes, and are called 'biaxial'. these include peridot, chrysoberal (alexandrite), topaz, and spodumene (kunzite) among the coommoner gemstones

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There is a point here which may usefully cleared up since it causes confusion to students. what is known as the critical angle of a stone is the angle at which rays of light passing from the stone into air are refracted at 90 to a line prependicular to the surface between the two media . rays striking the inside surface of a stone at angle creater than this critical angle cannot emerge, and are totally reflected back into the stone. here the stone is the denser medium, and the higher the refractive index of the stone is the smaller is its critical angle. in the case of refractomemter 

the principle is exactly the same, but here the glass of the refractometer is the denser medium, and the stone is the rarer medium. thus, in this case the higher the refractive index of the stone the greater is the critical angle between it and the glass of the instrument.

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When a doubly refractive stone is too small or has to low a birefringence to show doubling at all easily by the straight forward lens inspections describe above, another simple but more sensitive technique may yield valuable information. this has been practised indipendently in the past by max bauer, G.O. Wild, G.O. crowning shield and R.K. Mitchell, and in recent years has been systematiesd 

into a routine process of visual optics by alan hodgkinson, who uses it with exceptional skill and success.  The method is best applied in a darkened room with a single light source, which may be either a clear electric bulb with 'C' filament or (more conveniently) a small 'slit' torch of the type produced by hanneman lapidary specialist (figure 3.2). the stone to be examined, if large or mounted with an open baking, can be held in the hand, or if small in tongs. When a doubly refractive stone is too small or has to low a birefringence to show doubling at all easily by the straight forward lens

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In some minerals the figures very somewhat; thus in zircon, while the blue, white, and golden 'fried' type most popular in moderan jewellery are almost constant in their birefringence of 0.059. many of the ceyclonese zircons. particularly the green varieties, have a much lower double refractions, sinking in some cases practically to zero. this peculiaity is unique amongst minerals, and will be discussed at greater length in the chapter on zircon. synthetic rutile has been manufactured in the USA 

(since 1949), and its anormous double refraction will be one means of identification. calcite, though not a gemstone, plays an important part in gemmological instruments in its optocally pure form of 'ice land spar', how, apart from the refractometer, can one test whether a given stone doubly refractive or not? the simplest method of all, which with a little practice is quite easily carried out, is to examine the stone carefully with a powerful pocket lens-say giving 8x to 12x magnification. for example, look

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6.-3 COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE

While ruby and supphire are the red and blue varieties of the mineral corundum, there are also white, yellow, pink and green supphires. emerald and aquamarine are the rich green and pale blue varieties of beryls, but there are also white, pink, and yellow beryls, and green beryl that do not rank as emeralds, tourmaline, quartz, zircon and topaz are further well-known examples of such 

polychrome minerals, and tourmaline in particular is well known for frequently appearing as parti-coloured crystals, the boundaries between the colours -(usually green and pink) being quite sharp either across the length of the crystal or concentrically in zones. the reason of such confusing behaviour can usually be explained by the fact that there are a limited number (eight to be precise) of elements that give rise to colour in minerals.

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The totally and the only partially reflected rays are projected by a lens system on to a transparent scale which is viewed through an eyepiece. the part of the scale illuminated by the totally reflected rays will be brightly lit, while the rest of the scale will be relatively in shadow. the scale is calibrated by the 

marks of the instrument so as to read direct in refractive indices, so that by simply observing the positions of the edge of the shadow on the scale the refractive index of the stone tested can be ascertained. Refractometer of the total reflection type were in use during the latter half of last century, but the first satisfactory low priced instrument for testing precious stones was devised by Dr. G. F. Herbert Smith in 1907. the more recently designed and very compact rayner refractometer figure 2.3 (a) has a truncated prism of dense glass in place of the usual hemisphere, but the principle of the instrument is exactly the same. the prism form was originally evolved to allow isotropic minerals to be use in glass. special anderson-payne

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5-2..DOUBLE REFRACTION AND DISPERSION

As would appear in a singly refractive stone such as diamond, each edge appears as a double line (see figure 3.1). a word of warning, however, should be uttered here; according to their crystal structure, all doubly refracting stones have either one or two direction in which only single refraction obtains. those gemstones belonging to the tetragonal, trigonal or hexagonal system of crystals have only such direction; one 'optic axis' as it called. these minerals, which are grouped together optically us 'uniaxal', include 

zircon, corundum (ruby and sapphire), beryl (emerald and aquamarine), tourmaline, and quartz (amethyst and citrine). the minerals crystallising in either the orthorhombic, monoclinic or triclinic systems have two optic axes, and are called 'biaxial'. these include peridot, chrysoberal (alexandrite), topaz, and spodumene (kunzite) among the coommoner gemstones.

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By Aamir Mannan (1)
In 1972 Rayner succeeded in producing a refractometer of quite a new type although it acts upon the same principles as all others. in place of the customary scale of the conventional refractometer, the user (having placed the usual small drop of contact liquid and the stone to be tested on the table of the instrument) sees through the eyepiece a blank screen crossed with the faint shadow edge or shadow edge due to the stone. the position of which will depend, as usual, on the refractive index or indices of the stone. by the turning the calibrated dial at the side of the instrument a black ribbon-like marker can be adjusted to coincide with each shadow-edge. the corresponding refractive index can than be read very easily from the dial hence the name 'dialdex'. reading accurate to one or two units in the third decimals place can be assured on this instrument rather more readily than on the previous standard Reyner refractometer. A further advantage has been gained by the employment

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6.-3 COLOUR, COLOUR FILTERS, AND THE DICHROSCOPE

While ruby and supphire are the red and blue varieties of the mineral corundum, there are also white, yellow, pink and green supphires. emerald and aquamarine are the rich green and pale blue varieties of beryls, but there are also white, pink, and yellow beryls, and green beryl that do not rank as emeralds, tourmaline, quartz, zircon and topaz are 

further well-known examples of such polychrome minerals, and tourmaline in particular is well known for frequently appearing as parti-coloured crystals, the boundaries between the colours -(usually green and pink) being quite sharp either across the length of the crystal or concentrically in zones. the reason of such confusing behaviour can usually be explained by the fact that there are a limited number (eight to be precise) of elements that give rise to colour in minerals.

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7.-1 REFRACTIVE INDEX AND ITS MEASUREMENT

Thus the ray AO is refracted along OA', apart from the small fraction of light which is reflected at O, as indicated by the dotted line. similarly, the ray BO is refracted along OB', as we consider rays which form greater angles with the normal, NOM, we reach an angle where the refracted ray only just grazes along the surface, OQ, between the two media. this 

angle is know as the critical angle, and all ray reaching O form the denser medium at angle greater than the critical angle are totally reflected back into the denser medium, it being physically impossible for any light form such rays to penetrate into the rarer medium. in the post, the angle COM can be considered as the critical angle of incidence; all the light s thus totally reflected along OC", and any other rays, such as DO, with a still larger of incidence, is, of course, also totally reflected. it is important to realise that the size of the critical angles will depend upon the relations between the reflactive indices of the denser and the rarer medium.

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8.-2 DOUBLE REFRACTION AND DISPERSION

Thus is must be realised that if, say, a zircon is cut with is table facet at right-angles to optic axis it may be necessary to tile the stone and make observations trough the bezel facets to see the double refraction well developed. also, as R. T. liddicoat has pointed out, in directions at right-angle to the optic 

axis of the uniaxial minerals, although the double refraction is at is greatest is cannot be directly observed, as the image due to the extraordinary ray is either directly in front or directly behind that the due to the ordinary ray. with zircon, sphene, and peridot the birefringence  is so strong that even the novice should have no difficultly in seeing the effect mentioned effect in quartz and corundum 

unless the stone are larger. (J.-P. Poirot, director of the paris gem-testing laboratory, has made the important observation that  for a given birefringence the doubling effects is more marked in stones of low R.I. than in those of high index.)

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REFRACTIVE INDEX AND ITS MEASUREMENT

(The Dialdex Refractometer)

In 1972 Rayner succeeded in producing a refractometer of quite a new type although it acts upon the same principles as all others. in place of the customary scale of the conventional refractometer, the user (having placed the usual small drop of contact liquid and the stone to be tested on the table of the instrument) sees through the eyepiece 

a blank screen crossed with the faint shadow edge or shadow edge due to the stone. the position of which will depend, as usual, on the refractive index or indices of the stone. by the turning the calibrated dial at the side of the instrument a black ribbon-like marker can be adjusted to coincide with each shadow-edge. the corresponding refractive index can than be read very easily from the dial hence the name 'dialdex'. reading accurate to one or two