Here is what Spongebob told me, which some of you may find interesting
"Batman insert is a circle, which immerse the half to some chemicals for dyeing, than put into the oven for sintering.
Once the process is done, the blue color shows up at half of the circle, but the material properties are different comparing to the black color side.
These factories make the numbers by laser ablation, this high energy laser beam evaporates the ceramic to achieve the engraving purpose.
But due to the different material properties (blue and black ceramic), the different evaporation rate induce the bump at the boundary, this is the bump you have seen on the insert."
