Boyd’s Bible Dictionary:
(hyacinth). Zircon, a vari-colored gem (Rev. 9:17; 21:20).
Concise Bible Dictionary:
The word ύάκινθος signifies hyacinth, and this, as a color, is a deep purple. In Revelation 9:17 the horsemen had breastplates of fire, jacinth, and brimstone, which seem to imply flashes of colored light. In Revelation 21:20 the jacinth garnishes the eleventh foundation of the heavenly Jerusalem. It is supposed by some to be the same as the ligure. The Greek word occurs in the LXX (Ex. 25:4; Ex. 26:1), but is translated “blue.”
Hyacinthus Orientalis
Jackson’s Dictionary of Scripture Proper Names:
hyacinth: blue