I am in the process of giving the cushions in my home a revamp. I am aiming for new covers and feather inners for all of them.
I made the latest pair from some plain, textured 'found' fabric. The fibres looked natural to me, but to be sure I did a burn test.
I took some of the fibres and held them over a flame. I am quite sure the fabric is not artificial, and think it is probably silk or cotton or a blend of the 2.
Whatever they are - it is working well with the lovely duck feather inners.