This Calibration Block is the one with a natural crack on its surface. 

It includes broad-brush cracks and thin line, because of the grinding and stress corrosion. 

The center of this block has been magnetized permanently. By watching or other methods, you can assess the detection.

Specification:

Material: 90MnCrV8

Surface Hardness: 63HRC~70HRC

Magnetization: 1000A DC center conductor magnetization

Standard: GB/T15822.2-2005 & ISO9934-2:2002