I have since received a reply from Aldo of the Rossella Junck Gallery, who I believe has also joined this forum.
His belief is that the item was purchased in Florence, c.1970 but there also was a Vincenzo Nason, glass maker, who died in 1948.
Apparently the 1kg weight (as pictured on their site - so that's what it is!) were "used" [made?] in Italy before 1960, but not since. He also believes this effect of 'black and aventurina' is typical of the 1930s.
He is checking further, so I hope he does get back.