Complex sentence: Little is known about polonium's properties, but it is considered to be a metal, although some sources, such as John Emsley's book "Nature's Building Blocks" refer to polonium as a metalloid.
Split into simpler sentences: Little is known about polonium's properties, but it is considered to be a metal. However, some sources, such as John Emsley's book "Nature's Building Blocks" refer to polonium as a metalloid.