Visual size difference between these two stones

Yes. I can start seeing a size difference at 0.2mm, which is exactly your difference.

Thanks distracts

Anyone else?

5.89 vs. 6.12 results in a surface area difference of 7.5% and a diameter difference of 4%. This difference is probably somewhat visible when loose but fairly difficult to see the difference once set. A lot of experts on this forum have mentioned 5% diameter (not surface area) differences as the amount where size differences start to become noticeable - particularly in loose stones. I've actually spent a lot of time searching for information on this very topic myself and even drew a bunch of different sized circles using Microsoft Visio to try and determine when I could start to see apparent differences (again, this becomes less noticeable once the stone has been set).

For reference, I compared 6mm (.8ct), 6.25mm (.9ct) and 6.45mm (1ct) circles to each other in various different layouts. The 6mm and 6.45mm circles (7% diameter and 13.5% surface area differences) were easy to pick out from each other when viewed at actual size, while the 6.25mm and 6.45mm circles were much more difficult (3% diameter and 6% surface area differences).

I'm no expert, but I personally don't think that the .2mm that often gets thrown around here as a "visible" difference is very accurate. .2mm represents a MUCH greater percentage of the stone's size when you're talking about a 5.5mm stone vs. a 7mm stone and it would be a lot easier to see the difference in the smaller stone than in the larger one.

A .2mm size difference between 2 marbles is easier to spot than in 2 golf balls.

Thank you Jkozlow3

If you held them right next to each other then yes I think you will see a difference. Once set and not right next to the other it really won't be as obvious.