The most common gems used as alternatives to diamonds in high pressure anvil cells are moissanite, cubic zirconia and sapphire. These gems are useful in situations where diamonds cannot really be used.
For example, it is difficult to study the compressional behaviour of diamond itself in diamond anvil cells, since the signal from the anvils seriously interfere with that from the diamond sample. Of course, this is also the case whenever the measured properties are close to those of diamond.
The table included in the Technical Note which you can download here compares the properties of these different gem anvil materials (after Xu J, Mao H K, Hemley R J, 2002 J. Phys.: Condens. Matter 14, 11549).
It is noteworthy that the highest pressures achieved to date in gem anvil cells are:
58.7 GPa for moissanite anvils
(c.f. Xu J and Mao H K 2000 Science 290, 783 and Xu J, Mao H K, Hemley R J and Hines E 2002 J. Phys.: Condens. Matter 14)
25.8 GPa for sapphire anvils, and 16.7 GPa for cubic zirconia anvils
(c.f. Xu J, Yen J, Wang Y and Huang E 1995 High Pressure Res. 15, 127).
Click here to download moissanite, cubic zirconia and sapphire – technical note 2013.