DARQ – The Crypt 100% Achievement Guide DLC

The first skull is in the next room. Once you've solved the puzzle, the cabinet to the right of the image will open. He returns with his head rolling on the floor and enters the cabinet. You just have to follow the path to get the first quartz skull.

Head to the elevator hall and go underneath. Then move the elevator up by activating the mechanism with Lloyd.
Once the lift is up, move your head to the left as illustrated in the picture.

Move the elevator down to clear headroom.
With your head, follow the path down to the right and you will reach the second Quartz Skull.

To get the last quartz skull, get to the bell room with Lloyd and the head.

Let your head enter this room and take the elevator down. With your head, follow the path to the right and you will find the last quartz skull.

There is a dedicated guide for this achievement, I recommend you check it out!

Thank you @tasselfoot for this video-guide.

The following image presents the hidden path:
I recommend that you take the lower left arrow, then the lower right and finally the upper right.

Once you have closed the door with the scissors (first puzzle in the game), place your head on the bridge and activate the mechanism.
 

Cross the bridge (go right) and activate the mechanism again. You should now be able to get to the next room and go behind the button to the small opening in the wall.

Go this way and you will find where the diamond skull is found.

VIEW Quartz Skull Collector y Jeweler achievements.