The Quartz Idol is an item found in the room with the occult floor painting that can be reached by following the corridor to the right side of the main entrance of the Mansion. It's a small female figurine made from quartz which holds a book and serves as a key for two specific locations: Florentine's Library and the Temple of the Morning Star (subsequently leading us to the Realm of Raquia).
Originally, the door to the afore-mentioned location of the statuette is protected by a green ward . After having opened the Sarcophagus in the Study and survived our first unpleasant encounter with Belial in Hawk's Prison, the seal will explode though, allowing us to proceed through the unlit corridor now open for exploration and eventually enter a dark room whose walls are characterised by numerous empty frames and blood splattered all over them. On the floor we spot a laboriously executed painting displaying occult iconography, which will spawn a major Yielder upon closer inspection. Lighting all the five candles that are placed around it, we summon a necessary evil whose defeat will yield us the Quartz Idol:
Adam: This [quartz idol] opened the door to the library. That thing in its hands must be a book then.
Examining the occult floor painting in the Mansion which is used to summon the Beast creature that holds the Quartz Idol key:
Adam: Someone spent a lot oftime on this.