According to trueart.info, "sable" was the name trappers made up to refer to the marten, a Russian animal closely related to the mink. Red sable is the name given to the brushes with the highest quality sable hair.
According to trueart.info, sable hair is preferred because it is thick at the bottom and thin at the top. This quality allows the hair to maintain its shape and a pointed edge. It also absorbs well, which is essential for watercolors.
Kolinsky, a particular breed of mink that used to be found on the Kola Peninsula of Russia, makes the best red sable brushes. The kolinsky is now virtually extinct in northern Siberia and the name refers to hair taken from the Asian mink found in Siberia, northern China and Korea.
The best hairs are taken from male Asian mink during the winter, when the animal has a thicker winter coat. The thick hairs from the middle and underside of the tail are the highest quality, says trueart.info.
There are restrictions in place on the trade of the marten. The German brush company da Vinci still claims to be trading with the Soviets and using real kolinksy hair. Many companies will not disclose information about their finest brushes, according to trueart.info.
There are brushes made from synthetic materials, generally nylon, to simulate sable hair. The synthetic hairs are cut to resemble the change in width of the mink hairs to create the tip. Nylon, however, does not absorb like real hair. Nylon also contains a lot of spring, which can be a drawback, according to trueart.info.