The Collaborative International Dictionary of English v.0.48:

Jacinth \Ja"cinth\, n. [F. jacinthe, L. hyacinthus. See
   Hyacinth.]
   See Hyacinth. --Tennyson.
   [1913 Webster]

WordNet (r) 3.0 (2006):

jacinth
    n 1: a red transparent variety of zircon used as a gemstone
         [syn: hyacinth, jacinth]

Easton's 1897 Bible Dictionary:

Jacinth
   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.