Gold like glass?
Revelation 21:18 ESV
"The wall was built of jasper, while the city was pure gold, like clear glass." http://bible.com/59/rev.21.18.esv
But maybe the Romans understood the concept. And if this kind of cup was used to detect purity – or warn of impurities that are present in what it contains – then the whole metaphor starts to get really interesting:
Just an idea; not going to start a new denomination over it. :)