Phyllite is a type of foliated metamorphic rock primarily composed of quartz, sericite mica, and chlorite; the rock represents a gradiation in the degree of metamorphism between slate and mica schist. Minute crystals of graphite, sericite, or chlorite impart a silky, sometimes golden sheen to the surfaces of cleavage (or schistosity).

The protolith (or parent rock) for a phyllite is a slate. Its constituent platy minerals are larger than those in slate but are not visible with the naked eye. Phyllites are said to have a "phyllitic texture" and are usually classified as having a low grade on the regional metamorphic Barrovian sequence.

Phyllite has a fissility (a tendency to split into sheets) and will form under low grade metamorphic conditions. Phyllites are usually black or gray. The foliation is commonly crinkled or wavy in appearance.

Phyllite chose to recommend Truthfulness as its character trait for the Little Rox.