Adam Clarke on Revelation 21:20

The fifth, sardonyx - The onyx is an accidental variety of the agate kind; it is of a dark horny color, in which is a plate of a bluish white, and sometimes of red. When on one or both sides of the white there happens to lie also a plate of a reddish color, the jewelers call the stone a sardonyx.

The sixth, sardius - The sardius, sardel, or sardine stone, is a precious stone of a blood-red color.

The seventh, chrysolite - The gold stone. It is of a dusky green with a cast of yellow. It is a species of the topaz.

The eighth, beryl - This is a pellucid gem of a bluish green color.

The ninth, a topaz - A pale dead green, with a mixture of yellow. It is considered by the mineralogists as a variety of the sapphire.

The tenth, a chrysoprasus - A variety of the chrysolite, called by some the yellowish green and cloudy topaz. It differs from the chrysolite only in having a bluish hue.

The eleventh, a jacinth - A precious stone of a dead red color, with a mixture of yellow. It is the same as the hyacenet or cinnamon stone.

The twelfth, an amethyst - A gem generally of a purple or violet color, composed of a strong blue and deep red.

These stones are nearly the same with those on the breastplate of the high priest, Exo 28:17, etc., and probably were intended to express the meaning of the Hebrew words there used. See the notes on Exo 28:17, etc. where these gems are particularly explained.