Labradorite, a variety of plagioclase felspar (q.v.), named from Labrador, where it occurs in great quantity in rocks of Archaean age. It contains about 53 per cent. of silica, 30 per cent. of alumina, 12 per cent. of lime, and nearly 5 per cent. of soda. It is generally grey, and exhibits a rich play of colours, chiefly blue and green, due to included fibres, for which reason it is sometimes used in jewellery. It is a constituent of basalts and diabases.