Calcedonio glass was invented around 1450 on Murano and was given this name because it imitated a natural variety of calcedonio, the zoned agate.
The secret of its preparation was later lost and re-discovered, because small details compromise its correct preparation.
Basically it is an opaline glass coloured with veins of various colors on a dark background, which looks a homogeneous dark red-brown in the light.
It is obtained by mixing shards of white opaline, colored opaline glass and crystal.