Tourmalines on Pink Mica

Tourmaline

Tourmaline forms long prismatic crystals, often striated vertically. There are, however, also specimens in compact aggregates and in fibrous-radiating masses. The colour varies according to the chemical composition: multicoloured (elbaite), black (schorl), brown (dravite), pink (rubellite), blue (indicolite), colourless (achroite), etc. It is a very hard, heavy, insoluble mineral, with a fusibility that varies according to the quantities of iron and magnesium (the darker it is, the more difficult it will be to melt).

They form in granitic and pegmatitic rocks, but also in some eruptive and metamorphic rocks. It is often found associated with various minerals such as beryl, zircon, quartz and feldspar.

Specimens with intense colours and strong transparency are mostly used in gemmology, others are used by industry to exploit their piezoelectric properties.

Mica

Mica is an aluminium potassium silicate.

Micas generally crystallise in the monoclinic system and form elastic flakes, light, irregular, easily cleaved and very variable in colour.

In most cases they form within igneous and metamorphic rocks, more rarely in sedimentary rocks.

In Japan, micas were already often used in the 17th century to make the colours of woodblock prints more brilliant and the images more luminous.