https://en.wikipedia.org/wiki/Alloy_wheel
Magnesium wheels were the first die-cast wheels produced, and were often referred to as simply "mag wheels." Magnesium wheels were originally used for racing, but their popularity during the 1960s led to the development of other die-cast wheels, particularly of aluminum alloys. The term "mag wheels" became synonymous with die-cast wheels made from any material, from modern aluminum alloy wheels to plastic and composite wheels used on items like bicycles, wheelchairs, and skateboards.
However, pure magnesium wheels are no longer produced, being found only on classic cars. Magnesium suffers from many problems. It is very susceptible to pitting, cracking and corrosion, and starts to break down in just a few months when exposed to moisture. Magnesium wheels require constant maintenance to keep polished. Alloys of magnesium were later developed to help alleviate some of these problems.
Magnesium in bulk is hard to ignite but can be ignited by a burning tire or by prolonged scraping of the wheel on the road surface following a puncture. Once lit, it is very hard to extinguish, being able to burn under water or in carbon dioxide, which are common extinguishing materials.
The mass of a typical magnesium automotive wheel is about 5–9 kg (depending on size).

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