  We propose a new microscopic model for the $\{001\}$ planar defects in
diamond commonly called platelets. This model is based on the formation of a
metastable stacking fault, which can occur because of the ability of carbon to
stabilize in different bonding configurations. In our model the core of the
planar defect is basically a double layer of three-fold coordinated $sp^2$
carbon atoms embedded in the common $sp^3$ diamond structure. The properties of
the model were determined using {\it ab initio} total energy calculations. All
significant experimental signatures attributed to the platelets, namely, the
lattice displacement along the $[001]$ direction, the asymmetry between the
$[110]$ and the $[1\bar{1}0]$ directions, the infrared absorption peak
$B^\prime$, and broad luminescence lines that indicate the introduction of
levels in the band gap, are naturally accounted for in our model. The model is
also very appealing from the point of view of kinetics, since naturally
occurring shearing processes will lead to the formation of the metastable
fault.
