The "radiant" is a special cut for diamonds. This cut is derived from the brilliant cut, but it has more facets than the brilliant cut ordinarily has. Developed by an American named Henry Grossbard, the radiant cut has exactly 70 facets, which give it its characteristic appearance.

Rhodium plating is a process by which the surface of a piece of jewelry is coated with a thin layer of rhodium. Closely related to platinum, rhodium is a very hard metal with a clear, white gleam. Rhodium is used to give a fresher look to the often grayish shimmer of white gold alloys. The layer of rhodium also frequently serves as a protective coating: for example, for items made of silver, because rhodium does not oxidize. Rhodium plating is applied to jewelry after it has been submerged in a galvanic bath.

Rhodolite is actually a pyrope, but has been given a name of its own because of its distinctive rose-red color. Pyrope belongs to the garnet group and was a fashionable stone in the 18th and 19th centuries. The name "rhodolite" is sometimes also used to describe rose-red almandines.

River is an old and now no longer widely used term which was formerly used to appraise the quality of diamonds. "River" is a term used to describe the color of a diamond. "River" is the highest level in the color scale and corresponds to "highly fine white." In the CIBJO's internationally recognized color scale, this level of quality is subdivided into "highly fine white +" and "highly fine white" and is labeled with the letters "D" and "E." The next lowest quality levels are Top Wesselton ("fine white") and Wesselton ("white").

When many individual diamonds are individually set, then connected very closely yet flexibly to one another, the result is a piece of jewelry that looks like a luxuriously flowing, sparkling river, hence the name of this technique for setting diamonds: "rivière" is the French word for "a flowing river."

Rubellite is a variety of tourmaline. It occurs in colors ranging from pink to red, sometimes with a purplish tinge. Depending upon its hue, a rubellite may also be described as a "red tourmaline" or a "pink tourmaline."

Ruby received its name because of its color: the Latin word "rubens" means "red." One of the world's most valuable precious stones, a ruby can often be much more precious than a diamond of equal size because immaculate rubies are extremely rare. The most avidly sought stones come from the mines of Mogok (Burma) and have a unique color called "dove's blood" - a special shade of red covered with a breath of blue. Thailand, Sri Lanka, and Tanzania are important sources of rubies. It wasn't until 1800 that the ruby was recognized as a member of the corundum group. Prior to that date, the word "ruby" was also applied to red spinel and garnet, or else all three were simply called "carbuncle stone." In the past, the ruby was regarded as a symbol of power, courage, and dignity. During the Middle Ages, it was widely believed that a ruby would darken whenever ill luck or misfortune was imminent.

The rundiste, also frequently written "rondiste," is part of a gemstone cut. It comprises the encircling girdle between the upper and lower part of the cut. On a brilliant-cut diamond, the facets which are immediately adjacent to this separating belt are referred to as "upper rondiste facets" and "lower rondiste facets." These are followed by the chief facets and by the table (on the upper side) and the point or culet (on the lower side).

The highly refractive rutile occurs in colors ranging from reddish brown to blood red and black. Although the stone has an almost metallic gleam, it doesn't play an important role as a jewelry stone. Rutile has a relatively low hardness. This mineral, however, occurs as inclusions in a variety of gemstones. When it occurs as an inclusion, rutile typically appears in needlelike or hair-like shapes which create gorgeous patterns and surprising lighting effects. Rock crystal and smoky quartz sometimes enclose delicate, straw-yellow needles or fibers of rutile known as "Venus' hair." Stones containing such inclusions were avidly sought in ancient times.