The development of microelectronic manufacturing technology has enabled researchers to produce ultra high purity materials. When gold was purified to the limits of the technology, it had the appearance of clear glass and was completely transparent.
This fact is a particularly interesting and a curious revelation if one considers a passage in the King James Bible that was written ~1,900 years ago: "...and the street of the city was pure gold, as it were transparent glass" (Revelation 21:21).
Hmmm...