Tension ring
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A tension ring is a type of finger ring with an inset jewel where the ring itself acts as the entire mounting and not just the prongs, bezel or other type of mounting found on top of most ring shanks. The ring acts as one large spring with tremendous clamping force which is opened up and clamped shut onto the diamond, holding it securely in place. Some rings are advertised as tension rings, and perhaps have that "tension look" but they have a bridge holding the ring together underneath the diamond. This is not a tension set ring.
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[edit] History
The first tension ring was created in 1981 by the German company Niessing. Niessing's original tension ring designs were very bulky, resembling a solid metal inner tube wrapped around the finger, in order to provide enough strength to maintain tension on the suspended stone. Steven Kretchmer was the first American adoptee of this design, and perfected the work hardening process which allowed his ring designs to be less heavy than the Niessing originals without sacrificing strength. Other ring designers followed Kretchmer's adaptations within a few years. Currently Niessing, Kretchmer, Danhov and Gelin & Abaci are the industry leaders in tension ring production.
[edit] Technical Information
A true tension ring will be very beefy and undergo special alloying, hardening and treatment processes for super strength. The original Niessing tension ring was constructed out of 18 karat gold (18 karat is 75% pure gold and 25% other alloying metals) and weighed 35 grams. The remaining alloys in the 18 karat blend were non traditional jewelry metals used to give it a much stronger strength. In 1987 Steven Kretchmer patented a proprietary platinum alloy called "Plat/SK" that has since been licensed to other companies such as Hoover & Strong that require a super hard platinum alloy for their jewelry such as tension set rings. Steven Kretchmer's advancements in alloying of metals allowed the modern day tension ring to drop much of it's weight without sacrificing strength. Currently Steven Kretchmer and Danhov produce the strongest tension rings which exert up to 50,000 psi (350 MPa) on the diamond. This amount of strength is not possible to be exerted on any gem with a Mohs scale of mineral hardness less then 9.0. In order to exert very high pressure on a gem of high importance such as a large diamond or a rare sapphire, the maker must ensure an even distribution of pressure on the gem. This requires careful calculations and inspection of the gem to ensure every facet mates perfectly with the ring. When the ring is being made, the metal is cold-worked and hardened, and then once the ring is set it is heat-treated for additional hardness.
Tension rings have also since been manufactured in other metals that are naturally strong such as titanium or stainless steel that do not require special alloying or manufacturing processes. A stone can be set as-cast with relatively little ease. These rings are usually reserved for more "fashion" type jewelry with semi-precious gems such as tourmaline, topaz or small diamonds.
A ring may only be called a true tension ring if the ring itself is the only setting, with the stone suspended unaided between the force of its two opposing sides.
