18K gold does not tarnish. At 75% pure gold, the alloy's high gold content makes it highly resistant to the chemical reactions -- oxidation and sulfidation -- that cause tarnishing in lower-purity metals like silver and copper.
Tarnish is the result of surface metal reacting with atmospheric sulfur compounds, oxygen, and moisture to form dark metal sulfides or oxides. Silver tarnishes readily; copper oxidizes. Gold, however, is chemically inert -- it does not react with oxygen or sulfur under normal atmospheric conditions at any temperature encountered in daily life. At 75% gold, 18K's high gold content dominates the alloy's surface behavior, making the piece effectively tarnish-free under normal wear and storage conditions. The 25% alloy metals (silver, copper, and trace metals) could theoretically react, but at 18K the gold matrix prevents meaningful surface degradation.
Under extreme conditions -- prolonged exposure to strong chlorine, acids, or industrial chemicals -- 18K gold can experience surface changes. But under the conditions of normal daily wear and proper storage, a well-maintained 18K gold bracelet will look the same in twenty years as it does today, with professional polishing as needed.