There are two basic types of gemstone settings: open settings and closed settings. Open settings are any type of setting that allows light to enter through the bottom of a faceted or transparent cabochon gemstone.
In a "closed setting" light can only enter the stone from the top. This type of setting is appropriate for opaque cabochon-cut stones and highly refractive faceted stones where light can enter through the crown and table of the stone and be reflected back to the observer from within.
Open Gem Setting Types
Prong Settings
Within the category of "open settings" or "ŕ jour settings" there are several variations. The most common variety for faceted gemstones is a prong setting (below, left), with either 3 or 4 prongs that hold the stone in place. This type of setting exposes the maximum amount of light to the sides and bottom (pavilion).
Carré Settings
A Carré setting is where the stone is seated directly over a light well (above, right), and the stone is set by raising (hammering) four "spurs" with a "graver" tool.
Tension Settings
A "tension setting" uses the metal's natural tendency to "spring" back to its original position to hold the stone in place. The metal is spread apart, and the girdle of the stone is seated into small grooves in the inside surface of the metal. This type of setting requires special alloys of metal that are strong enough to create and withstand the necessary pressure to hold the stone firmly. Tension settings are only appropriate for very hard stones (Hardness of 9 to 10: diamond, ruby, sapphire, cz or moissanite) as the setting can exert up to 12,000 lbs. of pressure per square inch on the stone's girdle.
The tension-setting was developed in the 1960s by Professor Friedrich Becker of Niessing in Vreden, Germany. Metallurgist Steven Kretchmer of New York (ring at top, center) holds several recent patents for his advanced techniques in heat-treating alloys to increase strength and elasticity.
Frustum Settings
A Frustum, or "Hollow Cone" setting is a conical or tapered setting constructed from sheet metal. The stone's pavilion rests against the inside of the cone and the outer edge of the cone's lip is bet over the girdle to secure the stone. A frustum setting can be open or closed.
Closed Gem Setting Types
Variations of the closed setting are the "bezel (gypsy) setting, channel setting, cluster setting, Kundan setting, frustum setting, Pavé, and the seamless tube setting.
Bezel Settings
The oldest and most basic gemstone setting is the "Bezel", derived from the French word "Biseau" meaning chamfered. Used primarily to set cabochons, a vertical strip of metal is formed to encircle the stone, then soldered to a metal base. The stone is secured by pushing and bending the bezel towards the stone using a burnishing tool.
Channel Settings
Channel settings are primarily used to set faceted gemstones that are straight-sided, or quadrilateral in shape (baguette or princess cut). The stones are aligned in a channel, sitting girdle-to-girdle. step-cut stones can rest on a track giving a "keystone" effect. Matching stones that are cut to a uniform size for use in channel settings are called "calibré-cut," as in "Eternity Rings."
Pavé Settings
A Pavé Setting is a tight grouping of identically sized stones laid across a flat, or convex surface, from the French word for "paved." The stones are held in place using three to six raised beads per stone.
Bibliography & Suggestions for Further Study on Gem Settings
Oppi Untracht, Jewelry Concepts & Technology - Complete Reference Guide 
. Doubleday
Steven Kretchmer, Tension-Set Diamond Rings . www.stevenkretchmer.com
Niessing, Tension-Settings . www.niessing.com
Lapidary Journal, The Lapidary Journal - Gem & Jewelry-Making Magazine . www.lapidaryjournal.com
MJSA, Manufacturing Jewelers & Suppliers of America . www.mjsa.org
