It will often work well with stones that have been heated by fire.
The clay core of bronze sculptures made by lost wax casting can also be tested.
Natural crystalline materials contain imperfections: impurity ions, stress dislocations, and other phenomena that disturb the regularity of the electric field that holds the atoms in the crystalline lattice together.
These imperfections lead to local humps and dips in the crystalline material's electric potential.
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