What is the origin of the idiom "acid test" and when to use it?

The idiom "acid test" has its origins in the field of metallurgy, where acids were used to determine the purity and authenticity of precious metals like gold. If a metal passed the "acid test," it remained unaltered by the acid and was considered genuine. In a broader sense, the phrase has evolved to describe a rigorous and definitive examination or assessment that is used to determine the true value, quality, or effectiveness of something.