No, some blends are ok. I've used a wool alpaca silk blend. Wool felts and alpaca felts. I don't think silk would felt on its own but came along for the ride with the wool and alpace. To felt, it needs to be yarn that is supposed to be hand washed. Then when we machine wash it, it felts.
Just make sure you don't get wool treated with a superwash process. When in doubt, swatch.