Grows on the Sacred Rock.
The crystals are available in three colors: Blue, Red, and Yellow, but the color of crystals on your Sacred Rock in the Forest is determined randomly at the beginning of the game and fixed for the rest of the game, so you have to ask friends for the other colors.

How to get the other color crystals

When your Modern House, Friends Hotel, or Museum is ready to be upgraded, the Ask Friends button will appear only if you don't have enough crystals, you can ask friends for the crystals by tapping the button.
You can also receive crystals from friends when you fulfill their item requests.