Peridot gemstone

by Manisha G

image of peridot gemstoneThe mineralogical name of this stone is Olivine. Peridot is the green gem variety of Olivine.

Chemical composition: Silicate of magnesium and iron. The properties of magnesium and iron vary considerably, giving in effect of an isomorphism series. The colour is due to iron which is part of the basic formula. The mineral is therefore said to be “idiochromatic” or “self-coloured”.

Crystal characteristics: Orthorhombic system. Habit generally prismatic and often flattened. Complete and well formed crystals are rare.

Varieties: Yellowish-green, bottle-green, leek-green, olive –green.

Physical properties:

Cleavage: Distinct in two pincoidal direction along the length of the crystal intersecting at 90 degree. The stones flaws readily if dropped or carelessly heated during setting.

Hardness: 6.50

Specific Gravity: 3.34

Lustre: Oily – Vitreous

Refraction & Refractive Index: Double, biaxial, positive sign, R.I.: 1.654 – 1.690; D.R.: 0.036

Dichroism: Difficult to determine if the stone is pale coloured. Twin colours yellow-green and green. Apparent doubling back facet edges when viewed through the crown of the stone with a lens helps identification.

Spectrum: Well coloured specimens of peridot show an evenly spaced group of three lines at 4530A°, 4730A°, 4930A°. In paler stones these are very faint and may be reduced to a single vague band, the first of the three, marking the beginning of the blue.

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