I read that the "natural" color of glass was typically a pale green due to iron oxide in the sand. Manganese dioxide was/is used to "de-colorize" the glass to make it clear, but I don't know if that was widely used in Venetian glass making 500 years ago. Is it possible that the slightly green translucent color in these old beads - as seen in the inner layer on a lot of 7Ls - was due to using "natural" glass that lacked the de-colorizer?

And if the slightly green glass was colored further by adding cobalt oxide to get blue glass, a slightly green tinged blue was the result, rather than the more pure cobalt blue color we all know and love?