Catalytic converters are made of a metal cylindrum containing a ceramic piece in its core. The ceramic monolith has a honeycomb structure which contains the valuable metals, including copper, nickel, cerium, iron and manganese. Small amounts of rhodium are also found within a catalytic converter. Rhodium, like platinum and palladium, is very rare and valuable.
Even though the monolith is the most valuable part, recyclers are almost always buying the full converter and extract the ceramic themself in order to avoid depreciation or fraud.