In Red Dead Redemption II, you can find this material through exploration, unlike a large majority of special materials.
You can retrieve the RDR2 Abalone Shell Fragment starting Chapter 2 of the video game.
Where to Find the Abalone Shell Fragment:
The shell fragment is inside an old house in Rhodes. The said house is several steps north from the sheriff’s office. You can see the house’s location on the map below.
We have also marked the location of the house on our RDR2 Unique Items locations map, at position 2.
Once you reach the said location, look for the house in the picture below.
To enter the abandoned house with white windows, you may have to break the door.
As you enter, the material is on the table, to your right.
How to Use the Shell Fragment:
After you find the Abalone Shell Fragment you can use it at the nearest Fence to craft the Bison Horn Talisman. For this talisman, though, you also need the horn of a Legendary Bison and a Silver Earring. Finally, this talisman costs $38.50.