Pramāṇa · Search · About · API

Minerals and Metals in Ancient India v2.c43.s1: Categorisation of Gems

Minerals and Metals in Ancient India v2.c43.s1

Language
English

Contents

Categorisation of Gems

v2.c43.s1.1

Kautilīya Arthaśāstra of the pre-Christian era presented a list of gems which has been mentioned by us earlier in chapter 5. The principal categories stated therein are colourless quartz (sphaṭika), red ruby (padmarāga), blue sapphire (indranīla), green beryl (vaidūrya) and yellow topaz (puṣparāga), apart from several minor ones. Diamond (vajra), pearl (muktā) and coral (pravāla) were considered separately.

v2.c43.s1.2

Over the centuries, the pattern of categorisation of gems did not alter very much. The thirteenth century text RRS (4.4-4.7) listed five principal gems or pañca-ratna: vajra, padmarāga, indranīla, puṣparāga and marakata (the last one is emerald, a deeper green variety of beryl or vaidūrya). During the Gupta era, the Br. Sam. (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

v2.c43.s1.t1.1

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.

v2.c43.s1.t1.2

Table 7.2 provides the list of principal gems which were known and used in the ancient world. These gem minerals have been 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 materials may have high refractive index and lustre but are usually colourless when completely devoid of impurities. Table 7.2 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 333-36) has explained that not less than 13 out of the possible 15 causes attribute colours to minerals and gemstones.

v2.c43.s1.t1.3

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 a nuisance in the gem Table 7. 2 Gem Categories of Importance in the Ancient World

v2.c43.s1.t1.4

| Name English/Sanskrit | Colour | Hardness (Moh's Scale) | Reasons for Non-white Colour |
| Diamond/vajra | usually colourless | 10 | Blue due to boron, yellow/green due to nitrogen |
| Corundum/kuruvinda | usually colourless | 9 | Pure Al₂O₃ is colourless. Trace impurities make it ruby or sapphire. |
| Ruby/padmarāga | red | 9 | Cr³⁺ replaces Al³⁺. Transition involving ligand field effect |
| Sapphire/indranīla | blue | 9 | Four different mechanisms including charge transfer; combination of Fe and Ti |
| Non-blue sapphire | | 9 | Orange padparadsca (Mg²⁺ and Cr⁴⁺ replacing two Al³⁺ 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). |
| Chrysoberyl/Cat's eye marjaraṣaka/laśunia | green/yellow | 8-½ | Green due to Cr and yellow due to Fe substituting in Be Al₂O₄ structure. |
| Spinel/Balas-ruby/bālasūryaka | rose-red | 8 | Pure spinel Mg Al₂O₄ is colourless, Cr³⁺ substitution makes it rose-red. Fe and Mg may further alter the colour. |
| Topaz/puṣparāga also puṣyarāga | Yellow | 8 | Pure topaz is colourless [Al(F,OH)]₂ SiO₄. Yellow colour is due to impurities — ferrous oxide, lime and alkali. Dichroic, also blue. |
| Beryl/vaiḍūrya | Faint green water colour | 7-¾ | Pure beryl Be₃ Al₂ Si₆O₁₈ is colourless. Faint green due to substituting Cr³⁺. This is allochromatic effect — transition involving ligand field. |
| Emerald/marakata panna | Deep grass green | 7-¾ | This is beryl containing more Cr³⁺ imparting green colour. Dichroic, the second bluish colour is due to iron. |

v2.c43.s1.t1.5

| Name English/Sanskrit | Colour | Hardness (Moh's Scale) | Reasons for Non-white Colour |
| Aquamarine/ pāribhadra | Faint blue to bluish green | 7-¾ | Bluish tinge due to iron. |
| Zircon/gomed | Cow-flesh/ colour | 7-½ | Pure zircon ZrSiO₄ is colourless. Reddish-yellow colour of Hyacinth is due to Fe³⁺, decolourised on reduction to Fe²⁺. |
| Garnet/tāmrya or tāmḍi | Various colours | 7-¼ | Family of minerals M₃²⁺ . M₂³⁺ . (SiO₄)₃. Colour depends upon chemical composition. Hessonite is confused with Hyacinth. Colour due to iron. |
| Tourmaline/ vaikrānta | Various colours | 7-¼ | H₉Al₃(B.OH)₂Si₄O₁₉ pure tourmaline is light coloured. Fe²⁺ and Cr³⁺ make it dark green, Mg makes it brown and Mn dark red. |
| Quartz/sphatika | Colourless | 7 | Pure SiO₂ is colourless. Yellow citrine due to iron; green chrysoprase due to nickel, violet amethyst due to manganese etc. |
| Agate/akik Carnelian/ Rudhirākhya | Light colour Red | 6-½ | Hydrated SiO₂ is colourless. Hydrated ferric oxide as impurity makes it brown or yellow. Heating causes dehydration and ferric oxide gives red colour to carnelian. |
| Olivine/Chrysolite/ Peridot/puttikā/ jabarjad | Bottle green | 6-½ | Pure Mg₂SiO₄ is colourless. Replacement of part of MgO by ferrous oxide makes it yellowish green. Mn²⁺, Ni²⁺ further change the colour. |
| Orthoclase/Feldspar Moonstone/candra-kānta; Sunstone suryakānta | Opalescent; play of Colour | 6 | Interference of light from thin tayers. |
| Jade/pilu | White green black | 5-½ - 7 | Green due to Mn and Cr and Fe²⁺. Black due to Fe³⁺. |
| Turquoise | Blue | 6 | 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). |

v2.c43.s1.t1.6

| Name English / Sanskrit | Colour | Hardness (Moh’s Scale) | Reasons for Non-white Colour |
| Lapis Lazuli/Ultramarine/rājāvartal lajhward (Glass has hardness 5-5-1/2) | Deep azure blue | 5-½ | The constituent minerals are Sodium-Calcium-Aluminium silicates with sulphate and polysulphide (S₃) anions. The 19 outer electrons of S₃ 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. |
| Pearl/muktā | Usually white | 3-4 | Iron, manganese etc. possibly complexed with organic matter, so that the colour is perishable on heat. |
| Coral/pravāla/ vidruma | red, black etc. | 3-4 | Iron, manganese etc. possibly complexed with organic matter, so that the colour is perishable on heat. |

v2.c43.s1.t1.7

industry. However this could be easily detected since the material glass has a low hardness of 5. On the other hand, distinguishing between zircon (7-1/2) and garnet (7-1/4) of similar colour and little difference in hardness required careful streak tests.

Linked texts (4)