Icelandic sheep have some interesting fiber characteristics...

For starters they have a dual coat. Meaning that they have two layers of fleece: thel, a soft and fairly short inner layer that is fine and curly, and tog, a tough outer layer that's long and heavily curled. When spun together, the fleece makes "lopi yarn." It can also be separated into the two different layers. Each sheep also has different amounts of both of these layers, so while one may have mostly thel, another could have mostly tog. We have been selecting towards a better balance of the two, but there's still quite a lot of variability within the flock and each sheep has unique and different fleece.

The second unique characteristic of Icelandic sheep is that they have a natural wool break each year which means you can roo them (pull off their fiber) instead of shearing them. This means that there is no blunt end to any of the fiber strands and thus no clingy hooks in your yarn. We've had a spinner use some of our raw fiber, and she said it cleaned incredibly easily, and dyed really well.