Marble is a difficult material with which to work, especially at the sizes for pen caps and barrels.  This extensive review of the Visconti Millionaire collection by Nancy Olsen at Quill & Pad speaks to all aspects of the pens’ design & manufacture.

Quick Facts Visconti Millionaire
Editions: convertible fountain pen in four color variations
Cap and barrel: Portoro Black, Valencia Cream, Rain Forest Brown, and Issoire Green marble
Nib: palladium “Dreamtouch”
Limitation: 988 pieces of each variation
Price: $1,995

http://quillandpad.com/2015/08/13/viscontis-marvelous-marble-pens-look-out-michelangelo/