The sparkle and glitter and gleam of platypus pens comes from the shininess of the plastic used and the interplay of light and their layer lines.
So many fountain pens have a smooth polished surface that it is surely the default surface. Platypus 3D printed pens come in surface patterns that are much more intricate and interesting than the default. A smooth finish _could_ be achieved by sanding and polishing away the layer lines, by filling the lines, or with some types of plastic by solvent smoothing. However, why fight against the layer by layer nature of 3D printing with complicated post-print processing when a far better result is had by taking advantage of it?
Compare the bright reflections of a classic smooth polished pen, the Parker English Duofold, with a couple of 3D-printed Platypus pens. The Parker pen is shiny, certainly (well, apart from my finger prints!), and you might say it ‘catches the light’. But the reflection is minimal and almost static. When the pen is rotated along its long axis the reflection stays almost exactly the same in intensity and shape (run the short movie). Contrast that with a Platypus pens. Each layer has multiple places at which the angle exactly right to reflect into your eye and so you see a glittering array of reflections. Move the pen in any direction and the pattern changes, with bright spots fading and growing: the pen sparkles and the reflections move. The 3D-printed pens play with the light brilliantly!
…and that’s not all!
The layers of a 3D-printed fountain pen also hide fingerprints—something that’s quite useful in an object that is held in the hand during use!—as well as scratches. Not that a Platypus pen is easily scratched: the PLA they are made from is harder than, for example, ABS plastic and so they are naturally scratch resistant.