noun
GeologyA lens-shaped body of distinct composition in a rock, especially an inclusion of feldspar or quartz.
Origin
Mid 19th century (in an earlier sense). From phaco- + -oid, after ancient Greek ϕακοειδής having the shape of a lentil or lens, in Hellenistic Greek especially applied to the lens of the eye (also recorded as a Greek word in Latin context in Vesalius).
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