Are chocolate diamonds expensive

"Expensive" is relative and depends on what you consider expensive.

Every diamond is valued according to its colour, clarity, carat weight and cut: the 4 Cs.

Chocolate diamonds -- a commercial description of the most common colour of diamond: brown -- are generally believed to have been formed by ". . . nickel impurities and lattice defects associated with plastic deformation. . . . " according to the Wikipedia entry for Brown diamonds.

Generally, you'll pay less for a brown diamond that you would for any other colour of diamond, given that the other three qualities are equal.