There is evidence that part of kimberlitic microdiamonds was formed at low P-T parameters corresponding to graphite stability. Microdiamonds were also discovered in ultramafic, volcanic, and different metamorphic crust rocks formed at low P-T parameters. The formation of these diamonds is probably caused by introduction of metasomatic fluids at metastable P-T parameters. In our experiments nanosized (70–80 nm) diamonds were synthesized at earth crust P-T parameters from fluid phase. Their presence was confirmed by Raman spectroscopy and electronic microscopy methods.