Commentaries:
Robertson's Word Pictures (NT)
Sardonyx (sardonuc), here only in N.T., white with layers of red, from sardion (red carnelian) and onyx (white); for sardius (sardion) see Revelation 4:3; chrysolite (xrusoliqov), here only in N.T. (Exodus 28:20), stone of a golden colour like our topaz or amber or a yellow beryl or golden jasper; beryl (bhrullov), again here only in N.T. (Exodus 28:20), note the difficulty of identification, much like the emerald according to Pliny; for topaz (topazion), here only in N.T. (Exodus 28:17), a golden-greenish stolle; chrysoprase (chrusoprasos), here only in N.T. (not in LXX), in colour like a teek, translucent golden-green; jacinth (uakinqov), of the colour of the hyacinth, a violet colour (Pliny), already in Revelation 9:17 like blue smoke, like achates in LXX; amethyst (amequstov), only here in N.T. (Exodus 28:19), of a violet and purple colour, more brilliant than the uakinqov. Swete sums up the colours thus: blue (sapphire, jacinth, amethyst), green (jasper, chalcedony, emerald, beryl, topaz, chrysoprase), red (sardonyx, sardius), yellow (chrysolite). But even so there is great variety in hue and brilliancy and in the reaction on each other. Clement of Alexandria argues that this variety illustrates the variety of gifts and graces in the twelve apostles. Possibly so.
Other Robertson's Word Pictures (NT) entries containing Revelation 21:20:
Revelation 4:3
Revelation 9:17
Revelation 21:19
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