It looks like a toy but it is not. This YoYoFactory Hubstack Afterglow...
A Ceramic Concave stainless steel BallBearing. These are replacement Bearings with concave profile and available in Size D.
The Dif-e-Yo KonKave Stainless Bearing centers the string while at the same time allowing it to move when needed for response. Size D (5x11x5 mm.). For H-Spin.
Dif-e-Yo KonKave Ceramic Size D (5x11x5 mm.) for H-Spin and SPYY.