There have always been two different types - the ones with the two arcs close are metallic outcrops (type 2) and the ones where the two arcs are spaced further apart are mineral outcrops (type 1). The metallic ones are usually lighter than the mineral ones but they both can be in different colours depending upon the body's makeup.
Have a look at the wavescanner website to see the different returns and the sounds associated with each:
http://www.wavescanner.net/
