The mottled, slightly marbled effect in the Björk rug is created by interweaving different colored wool yarns in the weft.
By the way, wool is not only known for its softness or warmth: Because its fiber contains oils, wool is naturally resistant to soiling, which for 'Björk' specifically means that the rug is quite a sturdy little fellow. Not entirely irrelevant if you have living at home, for example, crumb monsters and spill aunts ...
'Björk' is woven by hand, so the rugs may have slight differences in color as well as appearance.
I have four different measurements of each color to choose from. Find the right one for you and your home.