Complex sentence: The Black Stone was originally a single piece of rock but today consists of a number of fragments which have been cemented together and surrounded by a silver frame, which is fastened by silver nails to the Kaaba's outer wall.
Split into simpler sentences: The Black Stone was originally a single piece of rock but today consists of a number of fragmented pieces which have been cemented together. They are surrounded by a silver frame which is fastened by silver nails to the Kaaba's outer wall.