Information on Tourmaline gemstone

by Manisha G

Tourmaline

types-of-tourmalie-gemstone

Chemical composition: The mineral is a complex boro – silicate of alkalies and metals.

Colour: 

Iron Tourmaline: Generally black

Alkali Tourmaline: red, green or colourless

Magnesium Tourmaline: yellow, brown or black.

These three divisions grade into each other.

Composition varies widely in different specimens and very complex isomorphism replacement is present. The different composition leads to differences in colour and in the physical constants. Colour depends to some extent on the chemical composition and not on accidental inclusion of impurities.

Crystal characteristics: Trigonal system. Prismatic habit, prisms are often three or six-sided and sometimes nine-sided striated parallel to their length and hemimorphic (the two ends differently terminated.) A rounded triangular outline in the direction of the main axis and the vertically striated prisms assist in identifying most tourmaline crystals. Some crystals are green inside and red outside. On vice-versa, or parti-coloured along the length of the crystal or other colours may also be involved.

Varieties: Tourmaline may be found almost in any colour. It is better to call all of them by the species name with a colour prefix, e.g. red tourmaline, green tourmaline, etc. Other names like rubellite for red or pink stones, indicolite for deep blues, sibertite for purple-red and archroite for colourless tourmaline are also used to-day.

Physical properties: Do not vary as much as might be expected, owing to the very complex nature of the isomorphism in this species.

Hardness: 7 to 7.5

S.G.: 3.05 – 3.15

Lustre: Vitreous

Refraction: Double. Uniaxial, negative sign.

R.I.: 1.622 – 1.641

D.R.: 0.013 – 0.024 (mean 0.018), can vary quite considerably.

Dichroism: In brown and green tourmaline dichroism is very strong. The ordinary ray is strongly absorbed in most crystals and often light is transmitted (the extraordinary ray) only across the prism, the crystals appear partially black and opaque down their length (along the ‘C’ axis). This has to be taken into account when cutting the stone. Stones showing such marked dichroism will not show doubling of the back facets because the ordinary ray is completely absorbed.

The polarity of tourmaline crystal is also displayed by the phenomenon of pyroelectricity shown by some crystals, which, when heated to about 100 degree C, develop positive electricity at one end of the crystal and negative at the other. This electrical charge enables the crystal to attract small pieces of paper or wood ashes.

It is also the reason why tourmaline collects dust, when kept in a heated shop window or museum display case, to a much greater extent than other gemstones. It must be mentioned that the iron-rich tourmalines, such as black schorl, do not exhibit this pyroelectricity to any extent.

Another property, also due to the polarity of tourmaline crystals, is that a charge of electricity may be induced when pressure is applied in the direction of the vertical crystal axis. This effect is termed piezoelectricity, and has been made use of in certain depth-recording apparatus for under-water craft.

Cut: Step, brilliant, mixed cabochon, also carvings. Dichroic character is so strong that each stone has to be considered individually from this point of view.

Detection: The possibility of yellow or pink tourmaline being mistaken for topaz has been discussed. They are close in constants and in colour. Other stones are sufficiently distinct in constants to give no trouble. The exception is the rare mineral andalusite which is an attractive reddish-brown or greenish gems. This stone is an attractive reddish-brown with glints of green dichroic colour, or green with reddish glints, depending on the way it has been cut from the crystal in respect to correspond to the upper limits of tourmaline. The birefringence (0.010) is, however, biaxial and much smaller than in tourmaline.

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