Tension ring
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A tension ring is a type of finger ring in which the ring itself acts as the mounting for an inset jewel. This is different from other ring designs, where a jewel is held on top of the ring shank by prongs, a bezel or other mounting. A tension ring acts as a single spring with tremendous clamping force, which is shut on the jewel to hold it securely in place. Some rings, while advertised as and having the appearance of tension rings, in fact have a bridge holding the ring together underneath the jewel. These are not true examples of a tension set ring.
It should also be noted that the term "tension setting" is, from the perspective of physical science, a misnomer. The stone is not held in tension (a pulling force) but in fact in compression (a pressing force.)
[edit] History
The first tension ring was created in 1981 by the German company Niessing. Niessing's original tension ring designs were very bulky in order to provide the strength to maintain sufficient pressure on the suspended stone. These early rings are so bulky that they resemble a solid metal inner tube wrapped around the finger. Steven Kretchmer was the first American adoptee of this design, and perfected the work hardening process that 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 tension ring is a very robust construct, having undergone special alloying, hardening and other treatment processes to increase its strength. When the ring is being made, the metal is cold-worked and hardened before setting the gem, then heat-treated for additional hardness once the gem is set.
The original Niessing tension ring was constructed out of 18 karat gold (75% pure gold and 25% other alloying metals) and weighed 35 grams. The alloys in the 18 karat blend were non traditional jewelry metals, used to give the ring much greater strength than normal. In 1987, Steven Kretchmer patented a proprietary platinum alloy called "Plat/SK". This has since been licensed to other companies, for example Hoover & Strong, who require a super hard platinum alloy for jewelry such as tension set rings. Steven Kretchmer's advancements in the alloying of metals allowed the modern day tension ring to lose much of its weight without sacrificing strength.
Currently, Steven Kretchmer and Danhov produce the strongest clamping tension rings. These rings exert up to 50,000 psi (350 MPa) on the diamond. It is not possible to exert so great a pressure on a stone with a Mohs scale of mineral hardness rating of less then 9.0. Furthermore, in order to exert this pressure on a gem, particularly one of high importance, the maker must ensure the even distribution of pressure upon it. This requires careful calculation, and inspection of the gem to ensure that every facet mates perfectly with the ring.
Tension rings are also manufactured using other metals, particularly those that are naturally strong, such as titanium or stainless steel. These materials do not require special alloying or manufacturing processes for the creation of a tension ring. In these cases, a stone can be set as-cast with relative ease. These materials are usually used for "fashion" jewelry, and are set with small diamonds or semi-precious gems such as topaz or tourmaline.
