Reference ST180009 identifies this pen as the Diabolo de Cartier Fountain Pen in black composite with platinum-plated trim and an 18k solid gold nib. In untouched, factory-sealed "New Old Stock" (NOS) condition with complete original packaging, its market value is $650 to $900+ USD.
Because it is the flagship fountain pen model rather than a ballpoint or rollerball, it sits at the top of the value range for this collection.

Model Specifications & Market Breakdown

Attribute Details
Official Model Name Diabolo de Cartier Fountain Pen
Reference Number ST180009
Materials Black composite barrel, platinum-plated trim, blue synthetic spinel cabochon on cap top
Nib & Filling System Rhodium-plated 18k (750) solid gold nib; uses Cartier ink cartridges or converter.
Untouched / Sealed Value (NOS) $650 – $900+ (uninked and still sealed in the factory Cartier bag with box and papers)