Heat Treatment

Some common terms used for heat treatment are baking, annealing, heating, browning, combusting, cooking, firing, toasting and roasting.

  • Nature of gemstone: Almost any gem material can be heated, except those that are affected by heat such as emerald and opal.
  • Materials used: Equipment can be a cooker, furnace, kiln, muffle, pot, oven, roaster, brazier etc. The heating agents used include gases, oils, charcoal, wood, coal, heating coils etc.
  • Possible effects: It is the process that causes alteration in physical and chemical structure of the gemstone by the use of heat thereby changing the optical properties like colour or clarity. A wide range of changes are being induced today by heat treatment.

    Among them a few important ones are as follows.

    S.No. Change Example
    1. Darken Color Light blue sapphire to dark blue sapphire
    2. Lighten Color Dark pink tourmaline to light pink tourmaline
    3. Color Change Amethyst to Citrine
    4. Removal of Secondary Color Removal of purplish hue from ruby, green color from aquamarine
    5. Development of Asterism Corundum
    6. Removal of Asterism Corundum
    7. Structural Change Low zircon to high type zircon
    8. Crackling Iris quartz, synthetic ruby
  • Conditions: The effects induce depends upon the following:
    1. The maximum temperature reached.
    2. The time for which the maximum temperature is maintained.
    3. The rate of heating.
    4. The rate of cooling.
    5. The chemical nature of the atmosphere
    6. The pressure of the atmosphere
    7. The nature of the material in contact with the gemstone.
    8. The quality of gemstone used.
  • Identification:
    • Magnification: The most useful and valuable data is obtained from inclusions and surface features. These are more clearly observed in corundum. The following features are seen:
      • Pock marked girdle.
      • Chatter marks or burn marks.
      • Diffused silk.
      • Partially melted inclusions.
      • Burst halos around certain inclusions.
      • Healed fingerprints.
    • U.V. Lamp: Some heat treated gemstones show fluorescence characteristics which are different and distinct from their unheated natural counter parts. Unless strong evidence from other sources suggests otherwise, such observations in ultraviolet lamp should be considered an additional clue, though not conclusive proof, of heat treatment.
    • Characteristic spectrums are seen in heated stones using U.V. or Infrared Spectroscopy.
    • Some gemstones show weak dichroism when compared with untreated natural counter parts.

Heat Treated Gemstones

  • Amber
    • Possible Changes: Darkening, Clarification, Sun-Spangled cracks.
    • Conditions: Temperature: 175 ± 25°C, Chemicals: Rapeseed oil.
    • Environment: Long but slow heating in sand.
    • Stability: Quite stable till heated again.
  • Beryl
    • Possible changes:
      1. Change yellow green to blue (aqua); yellow to colourless; orange to pink.
      2. Change pink to colourless.
      3. Remove maxixe blue; blue to pink, green to yellow.
    • Conditions: Temperature: 450°C ± 50°C; Environment: oxidizing atmosphere.
    • Causes: Iron is present within berylin two types of locations.
      1. One type is located on an aluminum site and gives a yellow colour if present as Fe3+. Heating produces the change from Fe3+ to Fe2+ and hence the colour changes from yellow to deeper yellow.
      2. The other type of iron is situated in a channel site and gives a blue colour that is unaffected by heating. If both types are present, heating changes green aquamarine to blue aquamarine.
      3. Heat also bleaches the colour due to a colour center in maxixe beryl.
    • Stability: All treatments are stable.
  • Chalcedony
    • Possible changes:
      1. Pale colors to brown and red.
      2. Pale colors to milky white.
    • Conditions: Ordinary temperature pressure conditions.
    • Causes: Basically hydration alteration, usually limonite to hematite.
    • Stability: Stable.
  • Corundum
    • Possible changes & Conditions:
      S.No. Change Temperature Atmosphere Time
      1. Lightening of dark blue. 1700°C Oxidizing 20hrs
      2. Darkening of light blue. 1770°C Reducing 6hrs
      3. Reduction of color zoning / yellow patch. 1800°C Oxidizing 3hrs
      4. Reduction of blue patch. 1800°C Oxidizing 4hrs
      5. Development of pink. 1800°C Reducing 3hrs
      6. Intensification of yellow. 1800°C Oxidizing 2hrs
      7. Development of blue. 1600°C Reducing 2hrs
      8. Removal of silk. 1650°C Oxidizing 2hrs
      9. Development of silk. 1500°C Oxidizing 2hrs
    • Stability: All are stable to ordinary temperature but a few fade in months.
    • Causes: The typical cause of most blue sapphires is caused by (Fe+2 - O - Ti+4) inter valence charge transfer (IVCT) process. Varying ratios of Fe+3, with Fe+2/Ti+4produce - blue / green, green / blue, to green and similarly appearing colours.

      Certain types of corundums contain in their crystals lattice

      • Various amounts of Titanium oxide (silk) and Iron.
      • When these corundums are heated at elevated temperatures under reducing atmospheric conditions, the Ti+4 atoms from the rutile are dissolved into the corundum crystal lattice, where the Fe+3 ions are already in solution.
      • As result pairs of ions form and produce blue colour.
      • In case of yellow sapphire the change is due to Fe+2 / Fe+3 pairs of unstable colour centres or traces of Fe+3 and colour centers.
  • Diamond
    • Possible changes:
      1. Alter the surface by burning.
      2. Change colour in chameleon diamond.
      3. Modify natural yellow colour.
      4. Modify irradiation colours to produce green, brown, orange, yellow, pink, red, purple etc.
    • Conditions:
      1. Warmed slightly with an alcohol flame or on exposure to light in case of chameleon diamond.
      2. High temperature and high pressure to convert type 1a to 1b or vice-versa (yellow colours).
    • Causes: Change in platelet structure, converts to bright yellow (Type 1a)
    • Stability:
      1. Chameleon diamonds very unstable even at room temperature.
      2. Yellow colour is stable.
  • Jade
    • Possible changes:
      1. Lightening green colour.
      2. Darkening yellow and brown colours.
    • Conditions: Varying.
    • Cause: Yellow / brown inclusions on heating get oxidized and become more dark.
    • Stability: All are stable.
  • Quartz
    • Possible changes:
      1. Amethyst (violet) to Citrine (yellow) or bicolor amethyst-citrine (ametrine).
      2. Amethyst to colourless to green.
      3. Smoky to paler to greenish yellow to green to colourless.
      4. Rose to lighter to colourless.
      5. Blue to modified or colourless.
      6. Yellow or brown to red-brown or red.
      7. Crackled for Iris quartz.
    • Conditions:
      S.No. Change Temperature Atmosphere
      1. Smoky to pale brown 280°C Oxidizing
      2. Smoky to blue green 280°C Oxidizing
      3. Smoky to colorless 400°C Oxidizing
      4. Amethyst to Citrine 450°C ± 50°C Oxidizing
      5. Amethyst to Colorless 600°C Oxidizing
      6. Pink to light pink 450°C Oxidizing
      7. Pink to colorless 550°C Oxidizing
      8. Deep blue to light blue 300°C Oxidizing
      9. Colorless to Iris quartz 300°C Oxidizing
    • Causes:
      • The cause of colour in smoky quartz is colour centre, equivalent to Al3+. On heating, the electrons are displaced and destroy the whole colour centers as all the electrons get paired with each other.
      • In case of amethyst, the cause of colour is somewhat similar. A transition metal ion (Iron) is the defect and can be removed on heating partially or completely.
      • In deep blue the colour is again due to a colour centre.
      • In pink quartz the cause of colour is due to Mn as an impurity. A change of valence state of Mn gives a light pink.
  • Topaz
    • Possible changes:
      1. Brown or orange to pink
      2. Yellow or green to colourless.
      3. Brown (Irradiated) to blue.
      4. Blue to brown.
    • Conditions:
      S.No Change Temperature Atmosphere Time
      1. Brown or orange to pink 500°C Ordinary Minutes to Hours
      2. Yellow or green to colorless 400°C Ordinary Minutes to Hours
      3. Brown (Irradiated) to blue 200°C ± 50°C Ordinary Minutes to Hours
      4. Blue to brown 450°C Ordinary Minutes to Hours
    • Causes: All colors, except pink, the color is due to color centers. There are two types of color centres. BFCC (brown fading color center) and BSCC (brown stable color center). In case of pink / orange topaz, the color is due to chromium as an impurity.
    • Stability: When BFCC topaz is exposed to sunlight for a few days, the color of the treated topaz fades, while others are stable to light.
  • Tourmaline
    • Possible changes:
      1. Red/pink to lighter shades to colourless.
      2. Purple to blue or dark green.
      3. Dark blue to light blue.
      4. Dark green to light green / yellow green.
    • Conditions: Due to the complex structure of tourmaline, the temperature for some colours varies from 260°C to 1000°C.
    • Causes: Complex, but most of the blue or green is due to Iron as an impurity, red or brown is due to iron and manganese together as impurities. Heating alters the valence state.
    • Stability: Stable to ordinary temperature / pressure conditions.
  • Zoisite
    • Possible changes: Yellowish brown / greenish brown to deep blue (tanzanite).
    • Conditions: Oxidizing atmosphere, temperature between 350° and 600°.
    • Causes: Heating alters the change of state.
    • Stability: Stable to ordinary temperature / pressure conditions.