Made of high gem quality, flawless diamonds, usually with 16 facets, they typically weigh 1 ⁄ 8 to 1 ⁄ 3 carat (25 to 70 mg). The culet (tip) is ground and polished to a hexadecagonal surface parallel to the table. The culets of the two diamonds face one another, and must be perfectly parallel in order to produce uniform pressure and to prevent dangerous strains. Specially selected anvils are required for specific measurements – for example, low diamond absorption and luminescence is required in corresponding experiments.