The nitrogen-vacancy center is a point defect in the diamond lattice. It consists of a nearest-neighbor pair of a nitrogen atom, which substitutes for a carbon atom, and a lattice vacancy.
Two charge states of this defect, neutral NV and negative NV, are known from spectroscopic studies using optical absorption, photoluminescence (PL), electron paramagnetic resonance (EPR) and optically detected magnetic resonance (ODMR), which can be viewed as a hybrid of PL and EPR; most details of the structure originate from EPR. The nitrogen atom on one hand has five valence electrons. Three of them are covalently bonded to the carbon atoms, while the other two remain non-bonded and are called a lone pair. The vacancy on the other hand has three unpaired electrons. Two of them form a quasi covalent bond and one remains unpaired. The overall symmetry, however, is axial (trigonal C); one can visualize this by imagining the three unpaired vacancy electrons continuously exchanging their roles.
The NV thus has one unpaired electron and is paramagnetic. However, despite extensive efforts, electron paramagnetic resonance signals from NV avoided detection for decades until 2008. Optical excitation is required to bring the NV defect into the EPR-detectable excited state; the signals from the ground state are presumably too broad for EPR detection.
The NV centers can be converted into NV by changing the Fermi level position. This can be achieved by applying external voltage to a p-n junction made from doped diamond, e.g., in a Schottky diode.
In the negative charge state NV, an extra electron is located at the vacancy site forming a spin S=1 pair with one of the vacancy electrons. As in NV, the vacancy electrons are "exchanging roles" preserving the overall trigonal symmetry. This NV state is what is commonly, and somewhat incorrectly, called "the nitrogen-vacancy center". The neutral state is not generally used for quantum technology.
The NV centers are randomly oriented within a diamond crystal. Ion implantation techniques can enable their artificial creation in predetermined positions.