Crystallographic studies of micro-inclusions in diamond coat, carried out by analytical electron microscopic techniques, show that structures possessing rhombohedral symmetry form a significant fraction of the population of well-crystallised particles. They are, however, less frequent than apatite or biotite, which were identified earlier by the same investigative methods. Thin-foil specimens of diamond coat were prepared by mechanical fine-polishing, and subsequently ion-beam milling, sawn and polished plates of coated diamonds oriented parallel to (100) or (110). The majority of crystalline inclusions analysed were ⩽0.2 µm in diameter. Data obtained on 15 individual inclusions, including composition analysis by energy-dispersive X-ray spectroscopy, are reported and discussed. The a-axis of the hexagonal unit cell, and the c/a ratio, were determined for all specimens, and are believed to be accurate to ≈1% in most cases. In 12 out of 15 specimens, the cations with Z ⩾ 11 identified comprised substantially only Mg, Fe and Ca. Values of a in these specimens ranged from 4.81 to 4.92 Å; and for three of them the space group R3¯, corresponding to the dolomite structure, was positively identified. These crystals are classed as ankerites. Representative Mg:Fe:Ca ratios in the ankerites are 28:18:54, with cell dimensions a = 4.87 Å, c/a = 3.37. The ankerites contained small amounts of Ba, typically 1–3%, and smaller amounts of Sr, typically 0.2–2%. Three out of the 15 specimens contained >10% Ba. In two of these, Ba and Ca concentrations were roughly equal, and in one of the two, which had cell parameters a = 5.11 Å, c/a = 3.50, the space group symmetry R3¯c, corresponding to the calcite structure, was verified.