A polariscope is a central instrument in gemology. Its primary purpose is to quickly reveal the internal crystal structure and optical properties of a gemstone by using polarized light. The instrument helps gemologists separate stones into the following three main groups:
Isotropic (singly refractive) materials: Light passing through the stone is not altered, causing the stone to remain dark (extinguished) at all angles when rotated. This is typical of amorphous materials and cubic crystals (e.g. glass, diamond, spinel and garnet).
Anisotropic (birefringent) materials: The stone splits the light into two beams. When the stone is rotated on the polariscope, it will flash alternately light and dark (four bright and four dark positions per revolution). This applies to ruby, sapphire, emerald, topaz and tourmaline, for example.
Aggregate: Consists of many small crystals grown together that can give a constant, evenly illuminated glow without flickering (for example, chalcedony and jade)
Structure
The instrument consists of two polarizing filters (a polarizer and an analyzer) placed one above the other with a light source at the bottom. One filter is fixed and the other is rotatable. When the filters are turned to a crossed position (perpendicular to each other), the light is completely blocked until a birefringent gemstone is placed between them, which rotates the plane of the light.
Areas of application
Authenticity check: Can quickly distinguish real gemstones from glass and other imitations.
Internal stresses: Can detect abnormal stress patterns (so-called "anomalous double refraction"), which often reveals whether a diamond is synthetic or whether a stone has undergone glass filling.
Interference Patterns (Conoscope): With an additional accessory (a conoscope), an experienced gemologist can see optical interference figures and determine whether the stone is uniaxial or biaxial.
