This is an exquisitely designer genuine black onyx women's cross necklace, from Apples of Gold Jewelry.
The Christian cross pendant measures approx. 1" tall x just under 3/4" wide and 1.75mm thick (1 1/4" tall with hoop/bale).
This onyx cross for women is also available in 14k yellow or white gold, shown below - as well as a matching men's version (also below).
"And whosoever doth not bear his cross, and come after me, cannot be my disciple" (Luke 14:27).