troctolite, coarse-grained, intrusive igneous rock that is composed almost entirely of olivine (often iron-rich) and plagioclase feldspar (labradorite or bytownite). The olivine may be wholly altered to serpentine, which gives the rock a stippled appearance of red, green, brown, yellow, and black spots on a pale background, reminiscent of the skin of a trout (in German, troctolite is Forellenstein, “trout rock”). Occurrences of this olivine-bearing gabbro include Wolimierz, Pol.; Oslo, Nor.; and Cornwall, Eng.