Optical Properties

The polariscope and dichroscope answer different questions. The polariscope tests whether a stone is singly or doubly refractive, which separates isotropic species (diamond, spinel, garnet) from all others at a glance. The dichroscope reveals how many distinct body colours a stone shows in different vibration directions; a tanzanite shows blue, violet, and bronze in three directions, while a synthetic blue spinel shows only one colour. Neither instrument requires a prepared surface, making them ideal first-pass tests that can be applied in seconds to any loose or mounted stone.

The refractometer simulator lets students practise reading the critical-angle shadow edge before working on a physical instrument. It renders realistic shadow positions for both the ordinary and extraordinary rays of doubly refractive stones, and explicitly models over-the-limit behaviour: when the RI of the stone exceeds the refractive index of the contact liquid, the shadow edge vanishes from the scale. This is the point at which the Hanneman/Hodgkinson shortcut (available in the Measurement section) must be applied. Understanding why the shadow edge disappears is core examination knowledge: the refractometer measures the critical angle of the stone-liquid interface, so a stone with RI above the liquid RI produces total internal reflection across the full scale.

The pleochroism reasoner performs a tolerant colour match against the pleochroism_color1, pleochroism_color2, and pleochroism_color3 fields stored in the mineral database for each species. Enter the observed colours (for example, red, orange, and yellow for hessonite garnet, or blue, colourless, and yellow for aquamarine) and the tool returns a ranked list of matching species. Pleochroism alone is rarely conclusive, but it effectively eliminates cubic species and focuses the investigation within a much narrower group.

The optic-sign reasoner handles both uniaxial and biaxial determinations. For uniaxial stones, sign is determined by comparing the extraordinary ray index (ε) with the ordinary ray index (ω): positive when ε > ω (quartz, zircon, corundum), negative when ε < ω (calcite, tourmaline). For biaxial stones, the sign depends on whether the β index lies closer to α (optically positive) or to γ (optically negative). The 2V angle (the angle between the two optic axes) is calculated via the Mallard cos²V⊂z; formula when all three principal indices are available. A large 2V (near 90°) places β midway between α and γ, making sign determination difficult and often requiring interference-figure observation under convergent light.

Working through these optical tools in sequence mirrors the systematic approach recommended in the FGA Diploma curriculum: polariscope to establish SR/DR character, dichroscope to count pleochroic colours, refractometer for RI readings, then optic sign to complete the optical description. Each tool explains the underlying principle, not just the outcome, so it functions as a learning aid as much as a reference calculator.