The eleventh of twelve precious stones used in the New Jerusalem (Revelation 21:20) is called in the Greek huakinthos (Strong's #G5192). Strong's states the word can be translated as either "jacinth" or "hyacinth." Thayer's lexicon equates the Greek word to "hyacinth."
Huakinthos is also translated as this precious stone in Revelation 9:17 where two hundred million horsemen are stated to have breastplates that partly look like, or have in them, this gemstone.
There is a difficulty, however, in determining what the Jacinth of Scripture looked like due to conflicting definitions regarding the color of the gemstone.
Both Strong's and the BDB lexicon do not define the coloration of the stone mentioned in Exodus 28:19. Strong's, however, does define the jacinth (hyacinth) gemstone mentioned in Revelation 21:20 as deep blue. Thayer's says the stone in Revelation 21 is "dark blue verging on black."
Keil and Delitzsch's Commentary declares the jacinth found in Exodus 28:19 is "a transparent stone chiefly of an orange color, but running sometimes into a reddish brown, at other times into a brownish or pale red, and sometimes into an approach to a pistachio green."
Easton's Bible Dictionary defines this gemstone as being a reddish blue or deep purple color. Both Fausset's and Smith's Bible dictionary states the stones are a red variety of zircon. Gill's Exposition of the Scriptures says the stone is either purple or violet.
Mindat.org, a popular mineral site, states that this stone (the reference of which is synonymous to hyacinths) can range in color from yellow-red to red-brown. The United States Geologic Survey site defines hyacinths as coming in yellow, orange, red and brown colors.
Amulets containing Jacinth were thought to protect travelers against the plague and any wounds or injuries they would otherwise experience during their trip. The stone was also thought to insure a warm reception at any inn he visited along the way and even protect the wearer from being hit by lightning (Curious Lore of Precious Stones, pages 81 - 82).