Unlike other special materials, you can find the Cobalt Petrified Wood through exploration.
The good news though is that you can retrieve the Cobalt Petrified Wood in Red Dead Redemption II, starting Chapter 2.
Where to Find the Cobalt Petrified Wood:
This specific material is inside a hidden chest, north-west of Lake Isabella. Below, you can see the Cobalt Petrified Wood‘s map location.
Look for it under a large rock, close to a cart and a frozen corpse. Below you can see the exact in-game location of the chest containing this unique item.
If you encounter difficulties, also check our Red Dead Redemption II Lock Boxes and Chests locations guide and you should see it on our map at location 19.
How to Use the Cobalt Wood:
After you find the Petrified Wood you can use it at the Fence to craft the Boar Tusk Talisman. For this, you will also need the tusk from a Legendary Boar, and a Gold Earring. Additionally, you have to pay $31.00.