In nature, impurities of all kinds are simultaneously present. For instance, iron adds a yellow tinge that turns a pure red ruby into an orange (padparacha) specimen that is cheaper. Similarly, when iron enters a blue sapphire, the yellow admixture produces green or blue green overtones that are not very desirable. Most sapphires are of this colour and attempts are made to change it to pure blue. Iron may give a ruby a brown tinge and the presence of titanium and vanadium in ruby turns it purple reducing the value of the stone.
Corundum is doubly refracting and a ray of white light passing through it breaks up into rays, each polarised in a different plane and travelling in slightly different directions, depending also on the colour of the ray. An object viewed through the stone therefore appears as a double image. As the stone is rotated, the colour of ruby and sapphire may also vary somewhat because of an effect called dichroism or pleochroism. Ruby and sapphire are therefore cut to exhibit the best colour a deep red or a royal blue when viewed from the top or table facet.
Seen through a magnifying lens, natural corundum exhibits inclusions that resulted from slow crystallisation in nature. The inclusions are generally like 'feathers' or fingerprints. Liquid inclusions looking like lace are also sometimes visible. A specimen without these inclusions would almost certainly be a synthetic stone or even a piece of glass. Sapphires also have a silky sheen caused by needles of titanium oxide (rutile). The rutile needles from Sri Lankan sapphires are longer and wider spaced than the stones from Burma. Montana sapphires show long rods or tubes ending in projections, while Australian sapphires have liquid filled cavities and dark flat cavities with strong zonal colouring.
The appearance of both sapphire and ruby can be altered by heat treatment in a proper environment. It is estimated that virtually all blue, yellow and golden sapphires are heated to intensify their colour permanently (see chapter on 'Gemstone Enhancement'). Except for white sapphire, all sapphires turn rich yellow to orange yellow when irradiated with gamma rays but the colour fades on exposure to light. Irradiation intensifies the colour of pale yellow sapphires, but the sapphires are unchanged or become a blue-tinged amber colour.
Synthesis of ruby and sapphire is done on a large scale, as large ruby rods are required to generate laser beams. Synthetic rubies and sapphires are very common in the market and many traders sell them as naturals in order to earn fast money. The synthetics show bubble-like inclusions under magnification and so can be distinguished from the natural gems. The better synthetics sometimes have feather-like inclusions, but shine abnormally under light as compared to natural stones.
A recent technical development gives ruby a rich red colour that penetrates into the stone and is not totally removed by polishing. Heavy elements like chromium can be diffused into sapphire and ruby, given enough temperatures and long enough time. The process apparently takes several days at a temperature of 1,600C, but the time reduces rapidly if the temperature is increased somewhat less high 2,070C, the melting point of corundum. Even at this high temperature, iron, titanium or chromium diffuses only to a few tenths of a millimetre in a reasonable time. Diffusion treatment is done on a cut stone and only light polishinng folllows diffusion and nickel diffusionn gives a yellow colour, while chromium and nickel together give a padparacha effect. These colours are all stable, even if the stone is heated. Apparently the diffusion can be detected by examining the stone in methyl iodide solution.
Titanium oxide and iron oxide mixtures are also deposited on a sapphire to crete a blue stone. The heating is done in a reducing atmosphere; sometimes after diffusion in air, the stone is heated in a reducing atmosphere to bring out the colour. This process was patented by Linde and is widely prevalent in Bangkok and Hong Kong.
The value of star rubies and sapphires in influenced by the intensity and attractiveness of the body colour and the strength, sharpness and uniformity of the star. Diffusion can be used to make a star gem out of a sapphire or ruby. Aluminium titanate is mixed with borax and silica as a flux and filler and painted on to the stone. The stone is then heated to 1,750 C for several days for the titanium to enter the stone is again heated to 1,300C to develop a good star. Fractured stones unsuitable for cutting are chosen for this treatment. The stars so produced are very sharp and of uneven colour.