Member of the Feldspar group.
Feldspars form complex isomorphous series between the endmembers albite and anortite on the one hand and the endmembers albite and orthoclase on the other hand.
Labradorite is the best known anorthite feldspar.
Blue labradorite from Madagascar
Origin of name: after the type locality (the place from which a mineral is first identified), Ford Harbour on St. Paul´s Island, Labrador, Canada, where the mineral was allegedly discovered by a Czech missionary in 1770.
Source: Wikipedia
Synonyms and trade names: blue and green labradorite is often marketed as "Rainbow Moonstone".
Labradorite which displays all colours of the rainbow is called "Spectrolite" in the gem trade.
Spectrolites from Madagascar
Can be confused with: clean or milky blue labradorite can easily be confused with blue moonstone. Spectrolites are quite unmistakable due their display of colour.
Localities: labradorite is found on all continents including Antarctica. Commercially important localities are Canada (Labrador), Finland, the Ukraine and Madagascar.
Handling: orthoclase cleaves perfectly in two directions and is quite brittle. Thus it must be handled and worn with care. Sensitive to heat, acids and brines. Do not clean ultrasonically, no galvanic bath!
Worth knowing: the iridescent display of colour in labradorite and spectrolite is called "labradorescence". It is due to the getting out of phase of light being reflected from parallel lamellae, similar to the iridescence produced by oil on water.