2. Categorisation of Gems (Ratna)

Kautiliya Arthas astra of the Pre-Christian era presented a in list of gems which has been mentioned by us earlier Chapter V. The principal categories stated there in are colourless quartz (sphatika). ein red ruby (padmaraga), blue sapphire (indranila), green beryl (vai durv and yellow topaz (pusparaga), apart from several minor ones. Diamond (vajra) pearl (mukta) and coral (pravala) were considered separately. Over the centuries, the pattern of categorisation of gems did not alter very much. The 13 th century text Rasaratna-samuccaya (4.4-4.7) listed five principal gems or panca-ratna: vajra, padmaraca, indranila, pusparaca and marakata (the last one is emerald, a deeper green variety of beryl or Vaidurva). During the Gupta era, the BS (80.2-5) listed 22 gems of which nine were principal ones (navaratna) representing nine planets (navagraha). Ancient Categorisation Viewed in terms of the Modern Science It is obvious that gems have been prized principally in terms of their lustre and colour. The ancient gemmologists realised that different minerals might have similar colour, and therefore identification of gem minerals involved at least one more parameter. Hardness of minerals was a very valuable guide in identification of gems, specially in view of the fact that the modern indices of evaluation such as specific gravity, refractive index, X-ray diffraction pattern etc. were not known in the ancient days. Table VII-2 provides the list of principal gems which were These ce minerals have been known and used in the ancient world. arranged in the decreasing order of hardness (Moh's Scale). Energetic chemical bonds, often of the covalent type, result in hardness, which in turn provides the necessary durability of gem minerals. Such

VII-5 materials may have high refractive index and lustre but are usually colourless when completely devoid of impurities. Table VII-2 shows how trace quantities of idiochromatic transition elements such as Fe, Cr, Mn, Ni, Co get into the hard but colourless networks and alter their colour drastically. Nassau (1983 23 and 333-336) has explained that not less than 13 out of possible 15 causes attribute colours to minerals and gemstones. the Naturally, different categories of gems may, and do turn out to have similar colours, and in such cases, determination of hardness of the sample by scratch test has been a very useful exercise in the identification of the gem. Gemmology has always involved the art and science of imitation, and detection of clever imitations. Coloured glass was invented in India to imitate beryl and other precious stones (Pliny 37.26) and therefore it became nuissance in the gem industry. However this could be easily detected since the material glass has a low hardness of 5. On the other hand, distinguishing between af reon (72) and gamet (77) of similar colour and little difference in hardness required careful streak tests.

Name English Sanskrit Diamond Vajra Corundum Kuruvinda usually Ruby Padmaraga Sapphire Indranila blue Non-blue Sapphire Chrysoberyl/Cat's eye Marjaraksaka Lasunia Spinel Balas-ruby/ Balasuryaka TABLE VII = 2 GEM CATEGORIES OF IMPORTANCE IN THE ANCIENT WORLD Colour Hardness (Moh's Scale) 10 colourless sara red green/yellow 8 1/2. rose-red Reasons for Non-white Colour VII-GA Blue due to boron, yellow/green due to nitrogen Pure A 1203 is colourless. Cr 03+ replaces Al 3+ ligand field effect Transition involving Four different mechanisms including charge transfer; combination of Fe and Ti 2+ 4+ Orange, padparadscha' (Mg and Cr *replacing two Al 3+ions); same colour (due to Ni and Cr); yellow Karundaka (only Ni); green (due to Co, V and or Ni); asterism or star sapphire (due to Ti). Green due to Cr and yellow due to Fe substituting in Be Al 204 structure. Pure spinel Mg Al is colourless, Cr 3+ substitution makes it rose-red. Fe and Mg may further alter the colour. Pure topaz is colourless [AL (F,OH)] 2 S 104. Yellow colour is due to impurities - ferrous oxide, lime and alkali. Dichroic, also blue Topaz Pusparaga Yellow

Name English Sanskrit Beryl Vaidurya Colour Faint green water colour Hardness (Moh's Scale) Reasons for Non-white Colour VII-6 B 74 Pure beryl Be 3 Al₂ Si 6018 is colourless. Paint green due to subs ti tuting Cr is allochromatic effect transition involving ligand field. This mle 3+ green colour. This is beryl containing more Cr imparting Dichroic, the second bluish colour is due to iron. Emerald Marakata Deep grass green. 7 Panna Aquamarine/Paribhadra Faint blue Bluish tinge due to iron to bluish green 72 Zircon Gomed mi Cow-flesh colour 7 Garnet/Tamrya or Tamdi Various colours 7 Family of als Tourmaline/Vaikranta Various Colours 7/14 Quartz/Sphatika Colourless 7 Pure zircon ZrSic is colourless. Reddish yellow colour of Hyacinth is dug decolourised on reduction to Fe + to Fe M 2+. M. (Si 04) 3 Colour depends upon chemical composition. Hessonite is confused with Hyacinth. Colour due to iron. 34 Hg Al 3 (B.OH)2 514019 pure tourmaline is light coloured. Fe 2+ and Cr make it dark green, Mg makes it brown and Mn dark red. Pure Sio, is colourless. Yellow citrine. due to iron; green chrysoprase due to nicke: violet amethyst due to manganese etc.

Name English Sanskrit Agate/Akik Carnelian/Rudhi rakhya Olivine/Chrysolite/ Peridot Puttika Jabarjad Colour Light colour Red Bottle green Hardness (Moh's Scale). HIN 161/1/2 N Orthoclase/Feldspar Moonstone/Candrakanta Sunstone/Survakanta Jade Pilu Opalescent; play of Colour 6 White green black 5 - 7 Turquoise Blue 6 Reasons for Non-white Colour VII 7 M Hydrated S 10, is colourless. Hydrated ferric oxide as impurity makes it brown or yellow. Heating causes dehydration and ferric oxide gives red colour to carnelian. Pure MS 104 is colourless. Replacement of part of MgO by ferrous oxide makes it yellowish green. Mn 2+, Ni 2+ further change the colour. Interference of light from thin tayers. 7 Green due to Mn and Cr and Fe²+ . Black due to Fe 3+ The mineral is a derivative for ortho-phosphoric acid, the hydrogen atoms replaced by radicals such as Al(OH)2. Fe(OH)2 (providing green tinge) and Cu(OH) (providing intense blue colour).

Name English Sanskrit Lapis Lazuli/ Ultra- -marine/Rajavarta/ Lajhward (Glass has hardness 5-5) Pearl Mukta Colour Deep azure blue Hardness (Moh's Scale) HIN Usually white 3-4 Coral/ Pravala/ Vidruma red, black etc. 3-4 Reasons for Non-white Colour VII-8 B The constituent minerals are Sodium-Calcium- -Aluminium silicates with sulphate and polysulphide (3) anions. The 19 outer electrons of S. 53 unit undergo transitions within the molecular orbitals. These produce absorption band in the yellow range and hence intense blue colour. Less sulphur in the mineral system gives green. colour which makes the gem less worthy. Iron, manganese etc. possibly complexed with organic matter, so that the colour is perishable on heat. 23