here's a super cool article about procedurally generated cloth with fiber-level geometric details! ~ http://www.cemyuksel.com/research/papers/real-time_fiber-level_cloth_rendering.pdf
it features a sweater model with over 20 million fiber curves (!!) rendered with different types of yarn and varied fiber geometry.
i think it's so fascinating how complex materials can be rendered at really small scales, especially when it results in anisotropic effects.