The number of sapphires you can earn from watching ads is limited. Whether you get a sapphire or diamonds is randomized.
If you have a chance to earn a sapphire, it will be mentioned in the list of rewards before you start watching the advertisements. When you've earned the sapphires you can earn at the moment, you will only see diamonds in the list of rewards.
If you are interested in more sapphires, you can always buy them with real money. Remember to ask your parents first.