Here's how you get them:
1. Obtain the Secret Key: You'll need the Secret Key to open the locked door in the Cinnabar Island Mansion. This key is given to you by Professor Oak after you defeat the Elite Four.
2. Enter the Mansion: Once you have the key, head to the mansion in Cinnabar Island. Use the Secret Key to unlock the door and enter.
3. Collect the Gems: Inside the mansion, you'll find two gems hidden in the following locations:
* Red Gem: This gem is found in a hidden room behind a bookcase. You can open the bookcase by talking to a Man on the second floor.
* Blue Gem: This gem is found in a room on the first floor. You can enter this room after you solve a small puzzle involving a bookshelf and a switch.
4. Return to Clefairy: After collecting both gems, return to the Clefairy you encountered in the mansion. Give her the gems, and she will join your team.
Important Note: This is how you acquire the gems in the FireRed version. In other Pokemon games, the methods for getting the gems and their locations might vary.