Put these gems in order of the number of vowels in each word

To solve this, we count the number of vowels (A, E, I, O, U) in each word:

Aquamarine: A, U, A, A, I, E → 6 vowels

Diamond: I, A, O → 3 vowels

Opal: O, A → 2 vowels

Ruby: U → 1 vowel