From the manufacturer:
Polyalchemy FX was born out of a desire to create more than just colored filaments.
We strived to deliver something a little different in hopes to spark creativity in our users.
This is only the beginning of the FX line and we will continue to experiment to bring you new and inspirational products in the near future.
The black spots in FX PLA are non abrasive and don't require a special hardened nozzle. We recommend a nozzle size of 0.4mm and above.