Diamond in the sky

Craft entered view from east, was moom less night could see milky way very good, object moved over base and took off to the north at high rate of speed to the heading toward caifornia city north of edwards.  
Went through milky way and blocked out all stars had a clear look at object,diamond shape, silent and black underbelly was diffinatley weird, scence I saw every thing else in A.F.  Inventory to that point plus classified aircraft. 
Aircraft were scrambled minutes later but it was gone.
Witness indicates that the date of the sighting is approximate.  PD