These paperweights are made by Fratelli Toso of Fondamenta Colleoni n. 7
30141 Murano (Venice, Italy).
The cube-like, blockish millefiori are usually formed into flower petals, leaves, fruits, vegetables, or a field of grass. Fratelli Toso's paperweights are quite unique. Once you know who made this style, you can easily identify their work. I've always found their glass pieces to be wonderful examples of the Muranese paperweight craft.
You can access the Fratelli Toso website at:http://www.fratellitoso.it