Crassula cremnophila was named by Braam van Wyk and the author in the magazine Aloe in 1999 (Van Jaarsveld & Van Wyk 1999) from plants collected in 1997 in the lower Kouga River region of the Eastern Cape. Named for its cliff-face habitat, from the Greek, kremno, meaning ‘cliff’ and phyton ‘plant’. Since its naming, it was found in a few more sites of the Kouga and eastern Baviaanskloof region. 

Crassula cremnophila belongs to section Rosulares, which includes 22 species (Toelken 1985). Rosulares are characterised, as its name implies, by plants with leaves in tight basal rosettes. The leaves all have the firm, bristle-like, hairy margin and the elongate inflorescence with a distinct peduncle. Plants vary from the dwarf C. socialis to C. acinaciformis, which can be more than a metre tall when in flower.

Crassula cremnophila is related to both C. montana subsp. quadrangularis and C. hemisphaerica. It is at once distinguished from these species by its glaucous leaves, hemispherical bodies and pink corolla, 7 mm long. The flowers of C. hemisphaerica are 2.0–2.8 mm long. The stamens of C. cremnophila are about 4 mm long and also without black anthers. Crassula montana subsp. quadrangularis is a smaller mat-forming species (square bodies) with white flowers. It is also related to C. intermedia from cliffs further east. The rosettes are also bluish green (glaucous) in the latter species but larger, and cup-shaped or sunken and never hemispherical as found in C. cremnophila. The inflorescence in C. intermedia is elongate with many side branches and the flowers are much smaller. Of the 23 species in section Rosulares, 10 are confined to cliffs, and all are dwarf cliff-huggers. These include C. brachystachya, C. cremnophila, C. cymbiformis, C. exilis, C. luederitzii, C. montana, C. pseudohemisphaerica, C. sediflora, C. socialis and C. tabularis. 

Of the other species commonly found on cliffs (but not obligatory) include C. capitella, C. orbicularis, C. intermedia, C. setulosa, C. tomentosa and C. barbata.

Crassula was established by Linnaeus in 1753 and the Latin genus name Crassula pertains to its thick fleshy parts: crassus, meaning ‘thick’, and ula, indicating diminutive, pertaining to the fleshy leaves (Clark & Charters 2016). The genus Crassula is very common all over South Africa with about 150 species. Helmut Toelken former South African botanist (presently Australian), revised the southern African Crassulaceae in 1985 (Toelken 1985). Crassula reaches their greatest species diversity in the semi-arid parts of the winter rainfall region of South Africa. Crassula today consists of about 200 species. The family Crassulaceae has a world-wide distribution consisting of 35 genera and about 1 400 species (Christenhusz 2017). The most cultivated is probably the jade or money plant, Crassula ovata, a thick-set South African species (from the eastern parts) which can reach a size of 3 m tall, also depicted in this series.

Fig. 6. Crassula cremnophila among lichen (Parmelia sp.) and moss. Note the old dried inflorescence.