You can measure most of the objects made of conductive materials, however, measurement errors will occur when the objects have rough and uneven surfaces.

You can measure the object which is made of a simple material.
You can still measure the object which is made of two or more materials affixed to each other as long as there is conduction between them.
You can measure the entire thickness of the object providing that a conductor is on the surface with an insulator at the core.
You can measure the entire thickness of the object (a conductor) with a flat surface and a spongy inside.
If the object has a spongy surface and a spongy inside, a measurement error will occur due to the uneven surface. In this case, please be noted that the measurement value will be an almost average of the thicknesses between the sunken and flat surface areas not just the thickness of the flat surface areas.