Diamond

1. Chemical Composition Carbon - C
2. Structure
  • Tetrahedral structure with each carbon atom, which is in the centre of a tetrahedron, covalently bonded to four other carbon atoms.
  • Bond lengths are 1.54 Angstrom unit giving rise to strong atomic bonding and therefore the hardness and stability in a diamond.
3. Classification / Type Diamonds are classified Type 1 and Type 2 on the basis of elements in the structure which give rise to varying properties / color. Types of Diamonds:
  • Type 1: Contains nitrogen which is subdivided into Type 1a and Type 1b.
    • Type 1a: Nitrogen as clusters. Most natural diamonds fall in this type. It is further sub-divided into Type 1aA and Type 1aB depending on the arrangement of nitrogen atoms with or without a vacancy.
    • Type 1b: Nitrogen is present in a single state as foreign atoms. Almost all synthetic diamonds and a very small of natural diamonds fall in this type.
  • Type 2: Contains hardly any nitrogen but contains other elements which are subdivided into Type 2a and Type 2b.
    • Type 2a: No appreciable nitrogen, generally brown, pink and colorless.
    • Type 2b: Contain boron and as a result such diamonds are semi-conducting. Generally blue diamonds.
4. Colors / Varieties
  • All colors (transparent to opaque)
  • Cryptocrystalline varieties: Bort, Carbonado, etc.
  • Chameleon Diamond: Golden yellow in the dark which changes to gray green when exposed to light.
5. Crystal System / Forms Cubic System / Variable octahedron, cube, dodecahedron, twinned macle and combination of cubic forms.
6. Hardness 10 (directional hardness variation)
7. Specific Gravity 3.515; crypto-crystalline carbonado 2.9 - 3.5
8. Cleavage / Fracture Perfect octahedral cleavage / step like fracture
9. Optic Character Isotropic
10. Lustre Adamantine
11. Refractive Index / Birefringence 2.417
12. Pleochroism Nil
13. Dispersion 0.044
14. Magnification
  • External features: Naturals, nicks, pits, scratches, polishing lines, damaged culet, etc.
  • Crystalline fingerprints or inclusions of Olivine, graphite, garnet, diopside, spinel, diamond, etc.; fingerprints
  • Structural inclusions: Twin planes (knots), grain lines, fine cleavage cracks (bearding) along girdle.
15. U.V. Fluorescence
  1. Colorless to yellow diamonds (cape series): Blue or yellow under ultra violet lamp. Natural colorless diamond exhibits blue under longwave while Synthetic colorless diamond generally exhibits blue under shortwave. Synthetic yellow diamond exhibits green in shortwave.
  2. Brown: Green to greenish yellow under ultra violet lamp.
  3. Pink: Variable; exhibits orange under ultra violet lamp.
  4. Some diamonds exhibit phosphorescence.
16. Cause of Color
  • Yellow: Nitrogen and related color centers.
  • Blue: Boron
  • Green: Color center due to natural or artificial irradiation.
  • Other colors are due to trace elements and related color centers.
17. Spectrum
  1. Colorless to yellow diamonds (cape series): Lines at 415nm, 423nm, 435nm, 452nm and 478.5nm.
  2. Brown: Lines at 503nm, 537nm and 495nm
  3. Pink: Line at 415nm and a diffused band at 550nm
  4. Treated yellow, green, brown, pink have characteristic spectrums.
18. Treatment (Enhancement)
  1. Coating / Spraying: Blue color is sprayed just below the girdle in yellow tinted diamonds to neutralize the yellow color and make it appear whiter.
  2. Colorless Impregnation: Fracture filling is done using lead glass to improve the clarity. It is identified by color flashes (blue / green / orange / pink) along cracks, diversification effects, etc.
  3. Lasering: With the help of laser rays, dark colored impurities are removed by drilling holes to the impurity and leaching it out. It maybe filled with lead glass and sealed and identified by color flakes along the drill hole. A new technique KM treatment expands the inclusion to the nearby cleavage crack / feather, which is then leached out with the help of acids. The treatment is virtually impossible to detect conclusively.
  4. Heating / Irradiation:
    1. Gamma rays produce a uniform blue-green color which on heating becomes dark green.
    2. Similarly using electrons, neutrons, protons, etc. various colors are obtained.
    3. Identification in case of cyclotron treated diamonds shows a dark ring within the girdle, umbrella effect (zone of color near the girdle).
    4. Spectroscopy is commonly preferred for the identification.
    5. Poor quality brown diamonds are exposed to high temperature of 900° to 1600° so that the diamond portions nearer to the cleavage / features turn to graphite giving black body color to the stone. The process is known as 'Graphitization' the treatment can be identified by observing the concentration of black / gray dots along the cracks or feathers.
19. Specific Tests & Remarks This test is applicable only to well proportion round brilliant cut diamond.
    • Read through effect: When placed table down on written matter and viewed from above the girdle, will not exhibit a read through effect.
    • This is due to the proportioning with respect to total internal reflection.
  2. Surface tension: This property is defined when 'Ink' is used. A line drawn across the surface with a diamond pen will remain as an unbroken line in a diamond but will bead up in a simulant.
  3. X-ray transparency: Transparent to x-rays due to lower atomic mass (atomic mass = 12). When x-ray is passed at 90° to the surface of the diamond it will pass through it but at angles other than 90° it will flourish which is due to the structure of diamond.
  4. Thermal Conduction::
    • Single crystal diamond varies from 1000 watts/m/°C for Type 1 to 2600 watts/m/°C for Type 2a.
    • This is measured with the help of a thermal probe and is also observed in the age old property of the Breath Test (when you blow on a diamond, the condensed moisture droplets evaporate quickly as compared to its simulants).
20. Synthesis Identification of Synthetic Diamonds
  1. Microscopy: Color zoning and graining (rectangular / square / octagonal), metallic inclusions (iron, nickel)
  2. U.V. Lamp: Green under longwave and yellow under shortwave.
  3. Spectroscopy: Moderate absorption band between 450nm and 500nm, weak bands between 470nm and 600nm.
  4. Reactions to diamond view and diamond sure.
  5. Types of Synthetic Diamonds
    • Synthetic Yellow Diamond: Type 1b
    • Synthetic Blue Diamond: Type 2b or mixed with Type 2b + Type 2a
    • Synthetic Colorless Diamond: Type 2a or mixed with Type 2a + Type 1b + Type 2b
21. Simulants (with key separation tests) Synthetic Moissanite (doubling, dispersion, inclusions), Zircon (optic character, spectrum, doubling), Strontium Titanate (dispersion, S.G., inclusions), Y.A.G. (S.G., dispersion), G.G.G. (S.G., lustre), Synthetic Rutile (optic character, dispersion, doubling), Natural / Synthetic Sapphire (optic character, lustre, dispersion), Doublets (inclusions, lustre).
22. Geological Occurrence Found in kimberlite (a type of peridotite rock), sometimes in lamproitic rocks. In pipe deposits, alluvial, eluvial and marine deposits.
23. Sources Africa (South Africa, Zaire, Botswana, Sierra Leone, Namaqualand, etc.), Australia, India (Madhya Pradesh, Andhra Pradesh, Orissa, etc.), China, Russia, Brazil, etc.
24. Cuts & Uses Facetted cuts, beads, etc.