The Greek word for the third stone is "chalkedon." It's a word that refers to copper or brass. It reminded me of the word study in Orichalcum from Ezekiel. I imagine that chalkedon might be a coppery-colored stone or a stone that shines similar to brass. There are actually quite a few Greek words in the Bible that have the same root (or so I understand - I could be wrong!).
Chalkos "brass" Strong's G5475 - 5 references to coins, the noise of brass instruments, other brass items
Chalkous "made of brass" Strong's G5470 - 1 reference in Revelation
Chalkeus "copper worker" Strong's G5471 - 1 reference in 2 Timothy
Chalkion "copper vessel" Strong's G5473 - 1 reference in Mark
Chalkedon "coppery stone" Strong's G5472 - 1 reference in Revelation
Chalkolibanon "fine brass" literally 'frankincense-brass' Strong's G5474 - 2 references both referring to Jesus' feet
Chasmal "brass or amber like metal" Strong's H2830 - all three references in Ezekiel regarding the vision of God's glory