Preparing an interference figure
Place the gem between crossed polarisers, locate a suitable orientation and observe through the conoscope sphere.
Uniaxial and biaxial patterns
A centred uniaxial figure may show a dark cross and concentric colour bands; a biaxial figure often exhibits separated, curved isogyres.
Avoid single-image conclusions
Apparent figures change with orientation, optic angle, strain and twinning. Observe movement across multiple orientations before interpreting.
Illustrated guide
Frequently asked questions
Do all gemstones show an interference figure?
No. The appearance depends on the material and whether a suitable optic-axis orientation can be found.