Materials such as glass stone are used to upgrade locations where the girls hang out. You can do this multiple times.

Upgrading a location will decrease the time until you can collect cash from it and will increase the cash amount and number of items rewarded from completing dates.

To upgrade a location, click/tap on the star icon before the location's name. If you have the required amount of materials, you can click on Unlock Now. If you don't have the required amount, you can pay the upgrade using gems.