These are Ceramic Block Magnets from the Magnet Source.
Not for Children!
Ceramic (ferrite) magnets are composed of strontium carbonate and
Charcoal gray in color
Most common grades of ceramic are 1, 5 and 8 (8 is the strongest)
Lower energy products than alnico and rare earth magnets
Lower service temperature than alnico and rare earth magnets
Metallic in appearance
Two ceramic block magnets
Length: 1-7/8" (48mm)
Width: 3/8" (10mm)
Thickness: 3/8" (10mm)