rhomboid vs rhomboidal

rhomboid is a noun but rhomboidal is not a noun.

Word	Noun	Adjective	Verb	Adverb
rhomboid	Yes	Yes	No	No
rhomboidal	No	Yes	No	No

As adjectives, rhomboid and rhomboidal are synonyms defined as:

rhomboid (noun)	rhomboidal (noun)
any of several muscles of the upper back that help move the shoulder blade
a parallelogram with adjacent sides of unequal lengths; an oblique-angled parallelogram with only the opposite sides equal

rhomboid (adjective)	rhomboidal (adjective)
shaped like a rhombus or rhomboid	shaped like a rhombus or rhomboid