pleochroism

Gemological properties including external observation with 10x loupe, hardness, internal observation, specific gravity, polariscopic study, pleochroism, absorption spectrum, ultraviolet light, Chelsea colour filter and refractive index were also determined.

Pleochroism is observed as the light passes through transparent media (Shurkliff, 1962; Berry et al., 1983).

Manning, "An optical absorption study of the origin of colour and pleochroism in pink and brown tourmalines," The Canadian Mineralogist, vol.

Blauwet loaned five of the faceted stones (0.52-1.96 ct; Figure 3) to AGL for examination, and the following gemmological properties were recorded: colour--brownish yellow to yellow, except for one sample that was orangey yellow; no obvious pleochroism; RI--1.592-1.623; birefringence--0.028-0.029; SG--3.12-3.20; fluorescence--inert to long-wave UV and weak-to-moderate yellow to short-wave UV radiation; and no features were visible with the desk-model spectroscope.

These xenoblasts usually show patchy pleochroism varying within individual grains due to compositional variations and irregular zoning patterns and also can develop twinned grains (Figure 6).

Amphibole phenocrysts (0.8-4.0 mm), with intense green to reddish brown pleochroism, cover a variety of compositions, probably as a product of a polybaric crystallization (Hammarstrom and Zen, 1986).

The pronounced change in color and pleochroism when crystals are heat-treated is generally thought to be caused by a valence change from [V.sup.3+] to [V.sup.4+], with a possible assist from [Ti.sup.4+] (Faye and Nickel, 1971).

It is most important to cut this gem correctly, due to its pleochroism, to take full advantage of the full violet colour

Raphides and druses had generally retained their high visibility and pleochroism (shifting colour zones) when viewed under cross-poladsed light.

Two types of amphibole are present, an earlier generation of magnesio-hornblende with greenish brown-brown-tan pleochroism and later pale green-green actinolitic hornblende.

The second generation of biotite typically shows more intensive pleochroism. Fibrous aggregates of sillimanite, which was formed together with muscovite, sometimes occur near muscovite tables.