culet

1999, John W. Kenney, III, 7. Pressure Effects on Emissive Materials, D. Max Roundhill, John P. Fackler Jr. (editors), Optoelectronic Properties of Inorganic Compounds, Plenum Press, page 234, The common diamond cut for high pressure work is the modified brilliant cut (see Fig. 1) in which the point of a brilliant cut diamond (i.e., the type of diamond commonly found in an engagement ring) is lopped off to provide a flat surface, called a culet, for pressure transmission. Two opposed diamond culets, separated by a thin, extrudable metal gasket into which a small sample hole is drilled, are squeezed together mechanically to reduce the volume of a hydrostatic medium and thereby increase the pressure of the sample as shown in Fig. 1.

2012, Samuel T. Weir, Yogesh K. Vohra, Chapter 4: Advances in Customized Diamond Anvils, John Loveday, High-Pressure Physics, Taylor & Francis (CRC Press), page 68, A final step is to polish the rough as-grown diamond layer on [the] culet of the designer anvil to smooth the surface and shape the culet so that is suitable for high-pressure experiments.