The rarest of all non-radioactive substances (and hence the most expensive precious metal), Rhodium is a lustrous, silver-white metal that has the important characteristics of being chemically resistant, insoluble in most acids, other than hot concentrated sulfuric acid, highly resistant to corrosion and has a low and stable contact resistance. Industrially, its prime use is as an alloying agent to harden its partner “noble” metals, Platinum and Palladium, but its rarity means that there is, increasingly, very little available naturally; recycling is critical to its continued presence, and use, in catalytic converters.