Katlang Pink Topaz: Pakistan
Chromium-coloured pink to champagne topaz from Katlang, Mardan district; Cr³⁺ colouring diagnostic, calcite-vein hosted, rare among topaz globally.
Introduction
Pink topaz from the Katlang deposit (Mardan district, Khyber Pakhtunkhwa) is exceptional among topaz varieties worldwide: its pink to pinkish-orange colour is caused by trace chromium (Cr³⁺) substituting into the topaz lattice, an extremely rare colouring mechanism for a species typically coloured by colour centres (irradiation), Fe, or Mn. Gübelin, Graziani, and Kazmi (1986) confirmed the chromium chromophore in this deposit, hosted in narrow calcite veins within recrystallised limestone. The Cr³⁺ absorption at ~550 nm and ~670 nm, detectable by UV-Vis spectroscopy and producing a weak reddish response on the Chelsea colour filter, distinguishes Katlang material from irradiation-induced pink topaz (different band pattern, unstable colour) and from Mn-coloured topaz.
This Cr-colouring means Katlang topaz will not fade under normal wearing conditions (a practical advantage over irradiated pink topaz, which may bleach in strong light or heat). Pakistan is the only major commercial source of Cr-coloured topaz. [1][2]
Geological Setting
Katlang mine geology:
- Host: Narrow calcite veins in recrystallised limestone (marble); the topaz crystallises within vein cavities
- Location: Katlang, Mardan district, Khyber Pakhtunkhwa; the deposit was documented by Spengler (1985) [2] and Gübelin, Graziani and Kazmi (1986) [1]
- Crystal habit: Well-formed prismatic crystals; sizes typically small (up to 3 cm reported); associated colourless, reddish-brown, and tan topaz also recovered
- Topaz species: Al₂SiO₄(F,OH)₂; the F/OH ratio at Katlang influences specific physical constants (RI, optic axial angle, unit cell dimensions)
Colour and Chromophore
What makes Katlang topaz unique:
Chromium Colouring
- Confirmed chromophore: Gübelin et al. (1986) confirmed "the color is due to trace elements, principally chromium (Cr³⁺)" [1]
- This Cr³⁺ substitution produces pink to pinkish-orange to champagne colour through absorption in the yellow-green region (~550 nm)
Colour Range
- Pink: Most prized; pure pink with slight orange modifier
- Pinkish-orange (peach): Transition colour between pink and champagne
- Champagne: Golden-yellow with slight pinkish cast
- Colourless, reddish-brown, and tan material is also recovered from the same vein system but lacks the Cr colouring
Distinguishing Katlang from Other Pink Topaz
| Property | Katlang (Cr-coloured) | Irradiated Pink Topaz | Mn-Coloured Pink Topaz |
|---|---|---|---|
| Chromophore | Cr³⁺ | Colour centres (radiation) | Mn³⁺ |
| Absorption spectrum | Cr bands (~550 nm, ~670 nm) | Different band pattern | Mn band (~490 nm) |
| Chelsea filter | Reddish (Cr response) | Inert or faint | May show weak response |
| Stability | Stable | May fade in light/heat [3] | Generally stable |
| Origin | Pakistan only (major source) | Any country's colourless topaz | Various sources |
| Treatment | Natural colour | Treated | Natural |
Chromium in Topaz
Physical Properties
Properties of Katlang topaz:
- Species formula: Al₂SiO₄(F,OH)₂; orthorhombic; biaxial positive
- RI: 1.619–1.627 (α), 1.620–1.628 (β), 1.627–1.636 (γ); birefringence ~0.008–0.010
- SG: ~3.49–3.57
- Hardness: 8 (Mohs)
- Note: Gübelin et al. noted the F/OH ratio variation at Katlang affects measured RI and optic axial angle compared to typical high-F topaz values
References
- ↑ 1. Gübelin, E.; Graziani, G.; Kazmi, A. (1986). Pink Topaz from Pakistan. Gems & Gemology, 22(3), 140–148. DOI: 10.5741/gems.22.3.140.
- ↑ 2. Spengler, R. (1985). The Katlang Pink Topaz Mine, North West Frontier Province, Pakistan. Journal of Gemmology, 19(8), 664–673. DOI: 10.15506/jog.1985.19.8.664.
- ↑ 3. Read, P. (2014). Gemmology (3rd ed.). Butterworth-Heinemann. DOI: 10.4324/9780080507224.