Spectrolite: Gemstone Information 

Spectrolite compared with labradorite from other sources is, subjectively, its greater range of colours, richness of colours and the number of colours seen at any one time. As a result all further sources of comparable material, regardless of locality, have also been termed ‘spectrolite’. Its composition is  An55, RI 1.56–1.57; DR 0.008; SG 2.69–2.70; hardness 6–6.5. Some labradorite of spectrolite quality has also been found in Madagascar.
• Crystal system: Triclinic
• Refractive index: α 1.554–1.563; β 1.559–1.568; γ 1.562–1.573 Biaxial +ve/ve
• Birefringence: 0.008–0.01
• Pleochroism: From the Congo, red stones show weak pleochroism whilst green stones have distinct greenish yellow to bluish green pleochroism
• Density: 2.68–2.71
• Hardness: 6–6.5
• Dispersion: Low (0.012), but the combination of a high polish and minute inclusions can simulate the effect of moderate dispersion
• Cleavage/fracture: Excellent {001}, very good {010} and good {110} cleavages/brittle with an uneven fracture
• Colour varieties: Red and green transparent labradorite from the Democratic Republic of Congo (see also andesine) have chemistry (An52 red–An55 green) and properties close to the labradorite–andesine boundary (An50). RI α 1.553–1.555; RI β 1.562–1.563; SG 2.68–2.70.
Congo labradorites have outstanding clarity, are of a deep reddish orange colour, and more deeply colour saturated than normally seen in the sunstone from Oregon which they strongly resemble. The cause of colour, largely due to copper, is complex and the intensity of colour is attributed to relatively high copper levels compared with that of Oregon material: Congo labradorites contain about twenty times more copper. The differences in colour are attributed to tiny copper colloids of different size.
• Inclusions: Exsolved black needle-like crystals of ilmenite are common, particularly in material that exhibits labradorescence where they, along with platy magnetite (which sometimes produce iridescent colours), impart a grey colour. When seen in association with rutile, are locality-typical for Malagassy labradorite moonstone. Transparent, blue flash, labradorite from Madagascar containing needle-like crystals similar to that in ‘black moonstone’ has been reported.
Zircon inclusions have also been reported in material from Madagascar. Platy hematite crystals, often exhibiting interference colours, are not uncommon. Tiny disc-like epigenetic exsolution crystals of native copper
are common in sunstone from Oregon. From the Congo, red stones are relatively clean with minor hollow
channels parallel to the twin lamellae and step-like healing fissures along cleavages. Tiny inclusions impart a milky turbidity. Greenstones,
under high magnification, were seen to contain parallel fluid channels and trails. Distinct lamellar zoning has been observed. Stones are described as rather clean with only some twin lamellae, hollow channels and fine undifferentiated inclusions
• Optical effects (chatoyancy, asterism, colour change, ADR): Labradorescence – play of colour produced by light interference from lamellar intergrowths. Labradorite is a grey feldspar showing signature labradorescence (a type
of iridescent schiller). Most common colours of labradorescence are blue and green. When many spectral colours are seen simultaneously the term ‘spectrolite’ has been coined.
In white incident light a milky turbidity with a reddish hue has been reported for greenstones from China. Colour change from green (daylight) to red in unspecified African material has been reported (rare): this should be considered alongside the ‘colour change’ reported for Chinese andesine and the milky turbidity with a reddish hue that has been reported for Congo material. No aventuresence has been reported in Congo material
• Absorption spectra (400–700 nm): From the Congo, visible spectra of red stones are characterized by a general increase in absorption towards the violet (attributed to copper) with distinct bands at 565 (attributed to Cu0
particles) and 380 nm (due to Fe3+). Visible spectra of green stones vary with pleochroic directions: the yellowish green direction showing a weak band at 430 nm (due to Fe3+) followed by strong absorption towards the violet (attributed to copper) and broad absorption towards the red with a minimum at 620 nm. The bluish green direction shows no distinct minimum at 620 nm and a broad absorption centred at 680 nm.
• Fluorescence: Rainbow moonstone shows an even, chalky, moderately blue fluorescence under LWUV. Under SWUV the intensity is diminished and fluorescence is a weak chalky pinkish orange. Under LWUV red stones (Congo) fluoresced weak to distinct orange and greenstones (Congo) appeared distinctly orange.