Determining the origin of the diamond using CPF (crossed polarization filters)
The CPF method, or crossed polarization filters, is a useful tool for distinguishing natural and synthetic diamonds. It is based on observation of birefringing in diamond.
Birefringing is a phenomenon where the light passing through anisotropic material is cleaved to two rays with different speeds and fracture. In diamonds, birefringing is caused by plastic deformation, growth sectors, dislocations and other crystal grid defects.
When observing between crossed polarizing filters, birefringing results in a color interference color, the intensity and pattern depends on the type and range of deformations in the diamond.
Natural diamonds show strong irregular birefringing, while synthetic HPHT diamonds are completely missing and CVD synthetic diamonds have typical regular patterns. By observing birefringing patterns under CPF, natural and synthetic origin can be distinguished.
Main patterns of birefringing in natural diamonds:
- Irregular spotted patterns in IA type diamonds
- Fine cross lines in type IIA diamonds
- Strong irregular birefringing in yellow and brown diamonds
Typical characters in synthetic diamonds:
- Missing or very weak birefringing in HPHT diamonds
- Regular vertical stripes at untreated CVD diamonds
- Sloping cross lines at CVD diamonds after HPHT adjustment
The CPF method is a fast and unimportant technique for initial diamond screening. However, it is necessary to use more advanced analytical methods such as FTIR or Raman spectroscopy to confirm synthetic origin. CPF provides a valuable clue to determine suspicious stones for further testing.
For a more detailed description of the methods to detect laboratory diamonds, see the following video:
