A number of interesting materials have been prepared in recent years by processes using very high pressures. Some of these materials are diamond, cubic boron nitride, dense silicon and germanium, coesite and stishovite silicas, metallic indium antimonide, and omega titanium. Diamond abrasive is now being manufactured and used on a sizable commercial basis. Cubic boron nitride, which is diamond-like, may prove to have commercial significance as an abrasive. The usual diamond-cubic forms of silicon, germanium, indium antimonide, etc. convert to metallic denser forms when subjected to very high pressure. Upon decompression these metallic forms revert to semiconducting forms more dense than the original diamond cubic form. When metallic indium antimonide is decompressed at liquid-nitrogen temperature the metallic form persists, and it may be studied and worked as long as its temperature is kept below about −60 C. At high pressures and temperatures silica crystallizes into forms more dense than quartz, namely, coesite and stishovite. These three forms have densities of 2.65, 3.01, and 4.35 g per cm3, respectively. Ordinary hcp titanium converts to a more dense omega (bcc) form at pressures exceeding about 90 kilobars, and this form is retained after decompression at room temperature. The retained omega form reverts to the usual hcp upon heating to a few hundred degrees C at atmospheric pressure.