Properly, the jacinth is a flower of a reddish blue or deep purple (hyacinth), and hence a precious stone of that color (Rev. 21:20).
It has been supposed to designate the same stone as the ligure (Hebrew: leshem) mentioned in Exodus 28:19 as the first stone of the third row in the high priest's breast-plate.
In Rev. 9:17, the word is simply descriptive of color.