The Stealth Ink mini marker has a 15mm nib and a short barrel. It has a pump valve that allows you to control how much ink is released into the nib. 

The marker is easily refillable by unscrewing the lid.

Stealth ink is totally colourless when first applied, but after a few hours the surface will become dyed a dark brown / black due to natural light influence.

Stealth ink is also very hard to buff!
 
Invisible Stealth Ink formula is enhances by the UV rays - best by sunshine or any other light.
 
The more your mark is exposed to UV rays, the darker it gets. 
 
Direct sunshine works the best, subway lights takes longer - same with summer and winter condition.