Calcedonio glass
Calcedonio is an Italian term for glass marbled with brown, blue, green and yellow swirls. The decoration imitates chalcedony and other banded semiprecious stones. It is glass, not a variety of natural chalcedony.
Venetian origin
The Corning Museum of Glass states that calcedonio was first made in Venice in the late fifteenth century. The term describes appearance and glass body, but does not by itself prove that a particular object comes from that period or from Venice.
Assessment
Examine the colour swirls through the glass, surface, pontil mark, shape, wear and any signatures. Ordinary marbled glass and later imitations may appear similar. Secure attribution requires comparison with documented works, not just a stone like pattern.
