All Treasures in Chapter 12 in Resident Evil 4 Remake

Gold Chicken Egg: After starting Chapter 12, take the lift back up to the room with the Gondola. Instead of using the Gondola, return to the Throne Room. In the back right corner, find a Square Lock Box. Use the Cubic Device to unlock it and obtain the Gold Chicken Egg.

Ornate Beetle: Return to the Castle Battlements and enter the first tower. In the corner behind the ladder, you will find a set of drawers. Use a Small Key to unlock them and collect the Ornate Beetle.

Gold Bar: Go to the Gondola Room, north of the Ballroom, and use the Gondola to reach the Clock Tower. Immediately upon entering, turn left and look behind some boxes to find a chest containing the Gold Bar.

Alexandrite: Proceed up the first two sets of stairs until you reach the first floor. Head towards the next set of stairs near a blocked path. Look up to find a container hanging from the ceiling; shoot it to get the Alexandrite.

Mirror with Pearls & Rubies: Ascend to the top of the Clock Tower. Pass the lever and look for a ladder you can climb down. Instead, find a hole in the handrail and proceed through it to find a chest containing the Mirror with Pearls & Rubies.

Extravagant Clock: Climb down the ladder mentioned in the previous step. Immediately to your left, you will find another chest containing the Extravagant Clock.

Yellow Diamond: Interact with the elevator's lever to ascend. You will need to fend off enemies that drop onto the platform. After reaching the top, go outside to find a container hanging from the scaffolding containing the Yellow Diamond.