there is a graphic bug leading to ugly stones. The problem occures on displays with higher PPI density if you are using Firefox 22 or newer. Stones seem to have no texture but have a strong line dividing the stone into a dark and a bright part.
I have a 15" screen with 1920x1080 pixels. I think it’s about 140ppi.
As workaround the Firefox option layout.css.devPixelsPerPx can be set from -1.0 to 1.0. This will fix the problem but also make all texts and graphics much smaller, so it’s only a workaround.
P.S.: Is it possible to upload pictures here?