Crystals are not secured to any of the CRYST bases.
Most people feel drawn to hold their crystals or carry them with them. Securing them to a base obviously won't allow you to do this, and it also makes it difficult to dust/clean the base.
If however, you have purchased a set and you would prefer them to be secured, you can secure them by using a double-sided adhesive tape.
There are a number of different brands that offer double-sided adhesive tape. Choose an adhesive tape that is clear and circular so that it won't affect the look of the crystals.