This is a Mexican sterling silver pendant, featuring an intricate representation of the famous Aztec Calendar stone (also known as the Sun Stone or Piedra del Sol). 

Design Features

• The Center: The central face depicts Tonatiuh, the Aztec sun god, surrounded by four square glyphs representing the four previous eras (suns) before our current world.

• The Rays: Points resembling large triangles point outward, symbolizing the sharp rays of light emanating from the sun.

• The Outer Rings: The bands concentric to the center contain complex glyphs representing the 20 days of the Aztec month

Hallmarks and Details

• Metal: Stamped "STERLING" and "925" (indicating 92.5% silver) alongside "MEXICO".

• Maker: The letters "EMF" represent the silversmith or workshop signature.

• Eagle Mark: The "Eagle 83" stamp dates the piece to roughly between 1948 and 1980, when the Mexican government used eagle symbols to denote silver purity and compliance.

Depending on the exact diameter (often around 1.75 to 2.25 inches) and the silver weight, authentic pieces with this level of detailed repoussé work usually retail between $60 and $150 on vintage jewelry platforms like eBay and Etsy.