1460478208-5b279d3d-1b38-4c42-9daf-787a9c6f6599

The invention presents a simple, non-destructive and non-abrasive method of diamond nucleation using polyethene. The two mould halves are held a small distance from one another so that a thin annular gap is formed between them, which gap is in communication with the mould cavity and through which gap excess material can escape. A black shading correcting portion corrects the image signal using a black reference level.