It is increasingly common for the catalytic converters on cars to be stolen. This is because the precious metal that forms a key element of the converters has become more expensive. That element is a rare one called rhodium which is, by weight, reportedly the most expensive element on the planet, beating out gold and silver and other precious metals. It is one of the rarest, just one part in a billion, compared with 5% for iron.
The converter on regular fuel vehicles is simple: a stainless steel shell surrounds a ceramic honeycomb monolith— that monolith is coated with three important precious metals: platinum, palladium, and rhodium.
As the car’s exhaust passes through this honeycomb the metals heat up and act as catalysts: turning carbon monoxide into carbon dioxide, unburned hydrocarbons to H20 and C02, and nitrous oxides into nitrogen and Carbon-dioxide.
Because these metals, and especially rhodium are so stable and durable they can perform this function over an extremely long lifetime of the car part—suffering very little loss in performance.