Well, this was very large, about an inch long, about a 1/4 of an inch thick in the middle, yellow, gelatinous. It was definitely not the normal urate portion of a dropping. I have read about the seminal plugs you describe, but haven't been keeping them long enough to have identified one. I sort of doubt this was one, unless they are much bigger than I imagine them to be.
:I'm assuming this was quite different from the normal white/yellowish urates (the white portion of the dropping) that reptiles and birds produce? If your skink is indeed male then what you found may have been a sperm plug. They tend to be yellowish and more formed, they become waxy as they dry. Owners of the somewhat larger male lizards like iguanas and beardeds often find these. Blue tongues produce them as well.