  We report the first experimental observation of a doubly-charged defect in
diamond, SiV2-, in silicon and nitrogen co-doped samples. We measure
spectroscopic signatures we attribute to substitutional silicon in diamond, and
identify a silicon-vacancy complex decorated with a nearest-neighbor nitrogen,
SiVN, supported by theoretical calculations. Samples containing silicon and
nitrogen are shown to be heavily photochromic, with the dominant visible
changes due to the loss of SiV0/- and gain in the optically-inactive SiV2-.
