The width of the vacancy related photoluminescence from diamond was used to qualitatively measure the strain in type-IIa diamonds of a range of brown colors and to measure the effect on the strain of high pressure high temperature annealing of the brown diamonds. The results indicated that for untreated type IIa diamonds the strain in colorless diamonds was generally less than that observed in brown ones. Annealing to remove the brown color brings about a measurable reduction in the strain as assessed via the luminescence line width; but a dependence on the depth of the original brown color was retained with the value remaining in most cases above 1.70meV, a width below which 92% of the untreated colorless diamonds lay.

The Vacancy as a Probe of the Strain in Type IIa Diamonds. D.Fisher, D.J.F.Evans, C.Glover, C.J.Kelly, M.J.Sheehy, G.C.Summerton: Diamond and Related Materials, 2006, 15[10], 1636-42