Learn French: L'alliage de cette devise est très rare. - The alloy of this currency is very rare.

Devise is a classic false friend! In French, une devise usually means "a currency" (especially a foreign one) or "a motto" (like the famous "Liberté, Égalité, Fraternité"). If you want to say "a device" in French, you would use un appareil or un dispositif.

How can I easily remember the gender of alliage?

Un alliage (an alloy) is masculine. You can safely guess this because almost all French nouns ending in -age are masculine. There are only a few common exceptions to this rule, such as une image, une page, and une plage.

Why do we use de instead of dans to say "the alloy in this currency"?

While an English speaker might naturally say "the alloy in this currency," French uses de (of) here. This is because de expresses that the alloy is an inherent characteristic belonging to that specific coin, rather than just a physical object resting inside of it.