Easton's Bible Dictionary: M. G. Easton, M.A., D.D.
Properly a flower of a reddish blue or deep purple (hyacinth), and hence a precious stone of that colour (Rev. 21:20). It has been supposed to designate the same stone as the ligure (Heb.leshem) mentioned in Ex. 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 colour.