David, not all black diamonds are treated. There are natural black
and grey diamonds colored by mineral inclusions, not the crystal
structure defects that cause other non-white diamond colors.
Facetable natural blacks, while not abundant, are found both in
kimberlites and alluvial deposits worldwide. =93Gems & Gemology=94
published an interesting study recently of black diamonds mined from
the Mir kimberlite pipe in Siberia. The article attributed black
color mainly to inclusions of magnetite, while greys tend to have
inclusions of hematite and native iron. They exhibited unusually
high magnetism which might be one factor in helping separate natural
blacks from treated.