I haven't seen documentation about origin of these. They come in various shapes and colors. I call them chunky frit aventurine beads. I believe larger chunks of frit (broken glass) are used to create the effect.

Most Ebay auctions call them Venetian. I've seen some very lively auctions for them in the past, especially the turquoise color. Times have changed however.

I want to believe they are Venetian based on the knotting and barrel clasp. I'm not sure however. Aventurine is used in the Czech bead industry too.

Beads with this type glass are made in a wide variety of shapes including flattened diamonds, long rectangles, rounds, large lenticular shapes, ovals. Colors I've seen include turquoise, all-aventurine, white, black, green.