"nl" already brought few points. As a practical test: take a piece made of cast magnesium (alloy) or cast aluminum (alloy). It'd be hard to easily tell each other apart, save for using a weak acid. Their strength is similar (esp. when alloyed, still worse off for the aluminum) but magnesium is non-trivially lighter. Here, a random quote [0]

Magnesium is also better at casting components with thinner walls and tighter tolerances than aluminum. However, even with the many advantages of magnesium, aluminum remains a less expensive alternative for die casting.

[0]: https://diecasting.com/blog/the-difference-between-aluminum-...