Here's why:
* Spanish vowel pronunciation: Spanish vowels are generally pronounced the same way regardless of their position in a word. This means that "u" in "ruby" will have its long vowel sound.
* "Rubí" in Spanish: The word "ruby" is spelled "rubí" in Spanish. The accent mark on the "i" indicates that it is pronounced with the long "i" sound.
Therefore, the "u" in "rubí" is a long vowel.