eli5 What determines whether a gem is a sapphire, ruby, emerald, diamond, etc if those gems don’t necessarily have to be the most common thought color? For example, sapphires can be colors other than blue. What’s the difference between a red sapphire and a ruby?

Question

eli5 What determines whether a gem is a sapphire, ruby, emerald, diamond, etc if those gems don’t necessarily have to be the most common thought color? For example, sapphires can be colors other than blue. What’s the difference between a red sapphire and a ruby?

In: 5800

Answer 1

As the top answers suggest, it’s the minerals that are present when the gemstone is formed.
The more heat and pressure, the more intense the color.

Bonus answer (I know you didn’t ask), colored diamonds are also determined due to the minerals present.
Yellow diamonds = Nitrogen
Blue diamonds = Boron