等色岩（Shonkinite）以普通辉石和正长石为主要组分的少见的深色侵入岩。其他矿物有橄榄石、黑云母和霞石，还有很少量的斜长石，不含有石英。在美国蒙大拿州海伍德山的尚金-萨格，等色岩形成层状岩盖（插入沉积岩层之间）之较大部分。

Shonkinite is a mafic foidal (feldspathoid bearing) syenite, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar in the form of sanidine, nepheline, augite, biotite, and olivine. Shonkinite is also used for mafic nepheline syenite with aegerine-augite as the pyroxene, and with the addition of plagioclase (andesine to labradorite). Nepheline in shonkinite from the type locality at Shonkin Sag in the Highwood Mountains of north-central Montana, from which the rock gets its name, is largely altered to natrolite and stilbite. The Shonkin Sag laccolith is a significant source of shonkinite.