Achyronychia is a monotypic genus of flowering plant containing the single species Achyronychia cooperi, which is known by the common names onyxflower and frost-mat. This plant is native to the Sonoran Desert of northern Mexico,  the Mojave and and the U.S. states of California, Nevada, Utah and Arizona. It is plentiful in Joshua Tree National Park.
Onyxflower is a diminutive plant which lies in a small mat flat on the ground. Its habitat is sand. It radiates several prostrate stems in all directions, each only a few centimeters long. The thick pale green leaves are paddle-shaped and under two centimeters long. In the leaf, axils grow dense bunches of tiny flowers. There are no petals, but each flower has five thin shiny white sepals that look like tiny fingernails. The the genus name Achyronychia is Greek for “chaff fingernail.”
An annual plant, it sports glabrous and/or hairy stems ranging from three to 17 millimeters. The onyxflower is taprooted. Its fleshy, ovate, scarious fringed white bladed leaves feature 0.1 to 0.2 millimeter stipules and a vein. Its inflorescence is axillary, with from twenty to sixty flowers, the pedicels of which range from 0.5–2.5 mm. The flowers are from 2.5 to 3 mm. The frost-mat’s hypanthia, that is, its floral cup, contains roughly ten ribs, with the fruit contained within it and the calyx abruptly expanded at the top.
It features five sepals. Sepals are the green portion of the plant that give protection to the flower in bud and then support the petals when in bloom. A typical plant possesses one to four fertile stamens and anywhere from 14 to 19 staminodes, which do not produce pollen. The staminodes are thread-like and positioned on the hypanthium rim. The minute fruit of the plant is utricle, dehiscent, and ovoid; with eight to ten teeth. The ovoid seeds are compressed, tan in color with a red dot near the narrow end and roughly one millimeter in diameter.
The plants are found on sandy slopes, flats and washes at elevations from 150 feet to 2,200 feet.