ZAMAC is an acronym for the Zinc, Aluminum, Magnesium, and Copper alloy that die-cast cars are made from.
The term "ZAMAC" is also used for an unpainted car, and/or base.
ZAMAC cars have been popular among collectors for some time. In 2004, almost half of the 100 First Editions were released with the ZAMAC look - clear coat over the bare metal. This was done intentionally. The first few were released in the vending machines at a 2004 convention. These could later be found exclusive to Toys R Us in specially marked cases. The cases in question had an aqua stripe on top of case opposed to the usual magenta stripe.
The photo below shows a ZAMAC or Unpainted Metal Base (chassis).