The crystalline structure of diamond may consist of C8, C10 or C12 building units. C8 was regarded as the building block of the hexagonal diamond also known as Lonsdaleite. Adamantane (C10H16) alicyclic hydrocarbon has the same arrangement of carbon atoms as the basic C10 unit of the cubic diamond lattice. C12 has five rings of mixed type, three of them in boat, two in chair conformation. Model building revealed that the C8 unit containing exclusively three rings in boat conformation does not exist. Further addition of carbon atoms to C8: a) results in C12 unit, b) allows the multiplication of C12 units, and c) by reducing the boat-to-chair ratio explains the hardness of the hexagonal diamond Lonsdaleite. The Lonsdaleite nucleus can be grown to special diamond grains with the outer atoms of the C12 building units replaced by different elements. This recognition can be utilized in the production of synthetic diamonds under high-pressure high-temperature conditions or in the chemical vapor deposition growth technique.