The sclera and cornea of the human eye both consist of collagen fibers, but the sclera is opaque and the cornea is transparent. Why?
That's an interesting question. First, think of the purpose. The goal of the sclera is different from that of the cornea. The sclera is designed to provide stability to the eye and protect it from damage. The collagen fibers that make it up are irregular in terms of thickness and direction, and this is what makes it opaque rather than clear (along with a concentration of pigmentation in the outer layer.)
The cornea is designed to act as a sort of lens to focus light on the pupil. It would not make sense for that lens to be "cloudy" because that would cut down on the amount of light able to enter. The fibers are more uniform in thickness and face the same direction. In addition, they have mechanisms to "pump" fluid away, keeping itself clear.
So that's about it...the sclera is opaque because it is not meant to let in light and doesn't have to be clear. The cornea needs to be clear to help focus the light. Here is an interesting, interactive look at the layers of the eye if you want to see how they work together.