Turquoise

Turquoise forms in arid environments where acidic, copper-rich groundwater percolates through aluminum-rich rock (typically volcanic or sedimentary). As this water evaporates in the dry climate, turquoise precipitates in fractures, cavities, and pore spaces within the host rock.

The process requires a specific cocktail of elements - copper provides the blue color, aluminum and phosphorus form the structural framework, and the right pH and temperature allow crystallization. This is why turquoise is found almost exclusively in arid and semi-arid regions: the American Southwest, Iran, the Sinai Desert, and central China.

The finest turquoise - the pure robin's-egg blue from Nishapur, Iran - forms when copper is the dominant coloring agent without significant iron contamination. As iron content increases, the color shifts from blue toward green. The dark veining seen in many specimens (called 'matrix') is the host rock in which the turquoise formed - limonite, sandstone, or other materials that filled adjacent fractures.

Turquoise's opaque blue to blue-green color and waxy luster are distinctive, but many imitations exist. Genuine turquoise has a Mohs hardness of 5-6 (depending on porosity), a white to pale green streak, and a waxy to dull luster on natural surfaces.

The most reliable field test is a hot needle: genuine turquoise won't melt or produce a chemical smell, while plastic imitations and resin-stabilized material will. Under magnification, natural turquoise often shows a slightly irregular, granular texture rather than a perfectly smooth surface. Matrix patterns in genuine turquoise are irregular and random - manufactured matrix (painted onto howlite or magnesite) looks too even.