Historically, tension rings were designed by a German jeweler Friedrich Becker by making use of a lateral pressure from a spring-hardened shank to keep precious stones. In these years, improvised methods were utilised to make sturdier compressions. Since a shank of an tension set ring does not meet, only hard gemstones like diamonds, rubies, sapphires, zirconium and cubic can be mounted and tensed. http://www.sitepromotiondirectory.com/detail/voltairediamonds-ie-2130256.htm