Conoscopic Observation and Interference Figures

• Polariscope (orthoscopic mode): Standard test – parallel polarised light; stone
  rotated to identify SR, DR, or ADR behaviour. Does not show interference figures.
• Conoscope (convergent mode): Converging lens added above or below the stone
  to bring a strongly convergent cone of polarised light to focus within the gem.
  The resulting interference figure is observed through a Bertrand lens or hand lens
  above the analyser.
• Bertrand lens: A small supplementary lens inserted above the analyser that focuses
  the back focal plane of the objective or converging lens – making the interference
  figure visible and centred.

If no dedicated conoscope attachment is available, a glass sphere (a ball bearing or
glass marble of any RI) placed above the stone in a high-RI immersion liquid acts as a
simple converging element. This improvised setup is described in standard Gem-A teaching
materials for use when a conoscope attachment is not available. Note: this improvised
method is widely described in gemmological teaching but a specific peer-reviewed paper
confirming its parameters was not retrieved during source verification – treat the
principle as established curriculum practice rather than a citable primary finding.

1. Select a clean, well-polished large flat facet (ideally the table or a large pavilion
   main facet).
2. Orient the stone so the suspected optic axis is approximately perpendicular to the
   facet being examined – this usually means examining the table facet of a well-cut
   stone.
3. For round brilliants: the table facet is a good starting orientation; for elongated
   stones (marquise, oval), try both the table and a large pavilion facet.

1. Set the polariscope to crossed polars (dark field).
2. Place the converging lens above or below the stone.
3. Insert the Bertrand lens above the analyser.
4. Observe the interference figure at low magnification (if using a lens system) or
   simply with the eye.
5. Rotate the stone stage and observe how the figure moves or remains stationary.
6. For optic sign determination: insert a λ-plate (first-order red plate) or quartz
   wedge and observe colour changes.

When the optic axis is perpendicular to the polished face, the centred uniaxial figure
shows:

• A stationary dark isogyre cross (Maltese cross) – the cross arms run north-south
  and east-west and remain fixed as the stage rotates.
• Concentric isochromatic rings surrounding the cross, corresponding to regions of
  equal retardation. Higher birefringence means more rings.
• The centre of the cross (where the arms intersect) is the melatope – the point
  where the optic axis emerges.

If the optic axis is not exactly perpendicular to the face, an off-centre figure results:
the cross arms sweep through the field of view as the stage rotates, rather than
remaining fixed. The melatope is visible at the edge of or outside the field of view.
An off-centre figure still confirms uniaxial character.

Insert a first-order red (λ-plate) accessory. The colour changes in the four quadrants
of the isochromatic rings indicate the optic sign:

• Uniaxial positive (+): addition colours (blue) appear in the NE–SW quadrants
  (along the slow-ray direction of the λ-plate); yellow in NW–SE.
• Uniaxial negative (−): addition colours in the NW–SE quadrants; yellow in NE–SW.

Important note: the exact quadrant labelling depends on the orientation convention of the
specific instrument and the direction of insertion of the λ-plate. The principle of
addition versus subtraction quadrants is standard crystallographic optics; verify the
quadrant convention against your specific polariscope's markings before reporting results.
Sturman & Parker (2010) provide a gemmological approach to optic sign determination:
Journal of Gemmology 32(1–4), 90–100 (DOI: 10.15506/jog.2010.32.1-4.90) [VERIFIED].

Observed when looking along the bisector of the acute angle between the two optic axes:

• Two melatopes visible in the field of view (or entering/leaving during rotation).
• Curved isogyres that form a "figure-eight" shape at the 45° stage position.
• As the stage rotates 90°, the isogyres sweep apart (from a crossed position at 0°)
  and then together again in the figure-eight pattern.
• The separation between the two melatopes estimates the optic axial angle (2V).
  Sturman (2007) provides a gemmological method for 2V estimation in biaxial gemstones:
  Journal of Gemmology 30(7), 443–452 (DOI: 10.15506/jog.2007.30.7.443) [VERIFIED].

Observed when looking along one optic axis (OA figure):

• One melatope visible; a single isogyre sweeps through the field of view during rotation.
• Useful for confirming biaxial character but less diagnostic than the Bxa figure.

Using a λ-plate at the 45° stage position (where isogyres are most curved):

• Biaxial positive (+): isogyre curves concave toward the acute bisectrix;
  addition colours appear in the concave region.
• Biaxial negative (−): isogyre curves convex away from the acute bisectrix;
  subtraction colours appear.

As with uniaxial sign determination, the exact interpretation depends on the instrument
orientation and λ-plate insertion direction.