In Colorado and Wyoming Diamonds have been found in 40 known diamond containing kimberlite pipes. Diamonds are very rare because the conditions suitable for their formation are also rare. The kimberlite pipes are vertical columns that were squeezed up from great depths(over 150 km deep) within the mantle, and these kimberlites have been deeply eroded (possibly 2000- 4000 feet) for hundreds of millions of years, so there must be plenty more diamonds within the nearby river and stream channels, but few have looked for them. The conditions suitable for the formation of a diamond is that it must be at extreme temperatures and pressures at great depth, and also carbon is needed because diamond is pure carbon. Diamonds have high symmetry and are isometric, and they have many crystal habits including, cubic, octahedron, dodecahedron. Diamonds have perfect cleavage in four directions that are parallel to ther octahedral faces. They are usually colorless but can rarely occur in many different colors. Diamonds are hydrophobic thus they repel water, they also are unaffected by acids. They usually concentrate in black sands in the river beds of the Wyoming and Colorade regions and can be panned for just like gold.
http://www.wsgs.uwyo.edu/Publications/OnlinePubs/docs/IP-12new.pdf
