If you take one of Pilot's Frixion pens and blur the boundary between a pencil and a gel ball pen you would wind up with one of these!
Pilot's Ball Enpitsu are my current pen fixation. Bold, vibrant somehow creamy colours that are erasable. Spotlessly erasable in fact, thanks to the Frixion technology. There are 24 colours in the range and the pens have a 0.7 nib which is thicker than I am used to but still great for my note taking and annotating requirements.
I imagine these would be great for students and ball pen doodlers alike. Sadly, they are not widely available in the UK yet. However, you can nab a set on eBay, Etsy and USA based stationers like Jetpens
*This set of pens was sent to me to review. The review is based on my honest and unbiased opinion after a period of using the pens*
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