It is similar to other subspecies of Calothamnus quadrifidus except that its leaves are linear and somewhat rough and scaly and the stamen bundles are relatively long.

petraeus is an erect or spreading shrub which sometimes grows to a height of 2.5 metres (8 ft) and lacks a lignotuber.

Flowering mainly occurs from August to October and is followed by fruits which are woody, roughly spherical capsules, 10–11 millimetres (0.39–0.43 in) long when mature.

The epithet is said to be from the Greek petraeus, meaning "rocky", in reference to the occurrence on rock outcrops.

petraeus occurs in the area between Karroun Hill Nature Reserve, Merredin and Yellowdine[1] in the Avon Wheatbelt, Coolgardie and Mallee biogeographic regions.