One of the joys of getting into vintage Rolex collectible games is discovering all sorts of design details. Many details can be found on the dial of the Rolex watch. Today, we talk about the origins of the Rolex sigma dial and what it means for the value of an old Rolex watch.
In the 1970s, a large number of Swiss watchmakers who were part of the L’Association pour la Promotion Industrielle de l’Or or APIOR formed an official initiative to highlight the use of gold on watches. 
Similar to how the “Chronometer” designation was controlled and guaranteed by Contrôle Officiel Suisse des Chronomètres to denote optimal timekeeping precision, PRIOR wanted to formalize how watchmakers signified the use of precious gold in watches – particularly important on steel watches since stainless steel and white gold are similar in color.
The symbol decided was the Greek letter Sigma: σ. According to a US Patent Office Registration Certificate the σ mark “certifies that the goods are of Swiss origin and contain solid gold up to a certain standard as established by the Swiss statutory requirements concerning the control of the use of gold and meet standards and quality established by the applicant.” It also indicates that the mark was “First used February 25, 1970; in commerce on or about Mar. 1, 1970.”
The Sigma mark could be decorated on the dial, bracelet, or case back of watches to signify the presence of gold. Some well-known replica watch brands that used the PRIOR Sigma mark included Patek Philippe, Vacheron Constantin, IWC, and of course, amazing Rolex. 
Sigma dials can be seen on fake Rolex watches dating from the early 1970s until the latter part of the decade; What’s more, Rolex Sigma dials have two σ marks surrounding the “T SWISS T” signature just under the 6 o’clock marker on the dial. So, Rolex Sigma dials denote that the hand-applied indexes and hands are craft from 18k gold. Rolex Sigma dials are more often found on stainless steel models, however, there are some examples of the Sigma dial on two-tone and yellow gold Rolex models as well.