Observations have been made on medium- and large-sized platelets in a natural type Ia diamond. Attempts have been made to detect any regularity in the structure of the platelets by means of electron diffraction, but none was found. The displacement associated with the platelets has been determined to be extrinsic, normal to the plane of the platelet and of magnitude 0.4 <010>. This value has led to the proposal of a model for the platelet structure in which the platelet consists of an interstitial layer of carbon atoms pentagonally bonded to the surrounding diamond matrix. The way in which the detailed bonding may be varied, while complete tetrahedral bonding is maintained, can lead to irregular structures consistent with the diffraction evidence. Possible mechanisms for producing interstitial carbon atoms are considered and the role of nitrogen in this context and in providing possible nuclei for platelet growth is discussed.