The Quartz Chunk in Red Dead Redemption II is an item you need for the Bear Claw Talisman.
It can be obtained starting Chapter 2 after you reach Horseshoe Overlook.
Where to Find the Quartz Chunk:
In order to retrieve the Quartz Chunk, you have to start the Stranger Mission named A Test of Faith. During this mission, you need to find the locations of several Dinosaur Bones for MacGuiness.
This unique RDR2 item is very similar to the Vintage Civil War Handcuffs because just like the latter it can be obtained by finding collectibles. In this case; however, you only need to find one.
So, first of all, make sure you find McGuinness north-east of Flatneck Station. After you talk to him, you will be asked to search for several Dinosaur Bones, and mark their locations in your journal and send them to the stranger.
After you find one of them, mail the location to MacGuiness from any post office.
Next, head out and fast-forward the in-game time, by setting camp and sleeping for at least 24 hours. Or you could spend those hours doing something else, cowboy.
Regardless, make sure you pay attention when you get a new mail or parcel. Head to the nearest post-office and pick up your reward below.
How to Use the Quartz Chunk:
After you receive it if you visit a Fence, you can to craft the Bear Claw Talisman.
Keep in mind though that for this talisman you also need the claw of a Legendary Bear (not common Grizzly Bear); and a Silver Chain Bracelet. You can try finding the bracelet while robbing Aberdeen Farm.