Here are some definitions of Onyx.
Note that all descriptions refer to striped material.
No mention is made of a solid colored material.

“Onyx” like so many other gem and and mineral names has been
misapplied and misunderstood for so long that the true meaning is
seldom recognized. The misapplication of terms is all too common
in our line of work. It has been interesting to see the many
definitions of onyx that have been posted.

Onyx \O"nyx, n. [L., fr. Gr. ? a claw, finger nail, a veined
gem. See Nail, and cf. Onycha.] (Min.) Chalcedony in parallel
layers of different shades of color. It is used for making
cameos, the figure being cut in one layer with the next as a

Onyx marble, a banded variety of marble or calcium carbonate
resembling onyx. It is obtained from Mexico.

Onyx a hail; claw; hoof, (Heb. shoham), a precious stone
adorning the breast-plate of the high priest and the shoulders of
the ephod (Ex. 28:9-12, 20; 35:27; Job 28:16; Ezek. 28:13). It
was found in the land of Havilah (Gen. 2:12). The LXX. translates
the Hebrew word by smaragdos, an emerald. Some think that the
sardonyx is meant. But the onyx differs from the sardonyx in
this, that while the latter has two layers (black and white) the
former has three (black, white, and red).

Steve Howard