Linde emerald is the trade name for hydrothermally grown synthetic emerald produced from approximately 1965 onwards by the Linde Division of Union Carbide Corporation. The Linde production was among the first commercially significant US-manufactured hydrothermal emeralds, paralleling earlier flux-grown emerald production by Carroll Chatham in San Francisco and the later production by Pierre Gilson in France. Linde emerald output ceased in the late 1970s, but the term remains in active gemmological use and Linde-grown stones continue to circulate in the secondary market.

Production method

Linde emerald was grown by hydrothermal synthesis, a process in which a charge of beryllium-aluminium-silicate nutrient is dissolved at elevated temperature and pressure in an aqueous solution within a sealed pressure vessel (autoclave), and the dissolved material is induced to deposit as crystalline emerald onto seed crystals at a slightly cooler region of the vessel. Typical conditions are temperatures of approximately 500 to 600 degrees Celsius and pressures of 1,000 to 2,000 atmospheres. Chromium added to the growth solution provides the characteristic green colour. The process produces relatively flawless single crystals at growth rates of approximately 0.3 millimetres per day.

Identification

Linde and other hydrothermal synthetic emeralds are distinguished from natural emerald by several diagnostic features. Refractive index falls slightly below natural-emerald range (typically 1.566 to 1.572 for Linde, against natural ranges of 1.566 to 1.602). Specific gravity is generally lower (approximately 2.66 to 2.69 versus 2.71 to 2.78 for natural). The Chelsea Colour Filter typically produces a brighter, more saturated red reaction in synthetic material than in natural emerald.

Inclusion patterns are more diagnostic. Hydrothermal synthetic emeralds, including Linde, frequently show chevron growth patterns, fine seed-plate ghosts, and characteristic two-phase fluid inclusions of a different morphology from those in natural Colombian or Brazilian emerald. UV fluorescence under longwave radiation tends to be stronger and more saturated red in Linde emerald than in most natural Colombian material, though Zambian and certain Brazilian emeralds can fluoresce comparably.

Trade significance

Linde and Linde-style hydrothermal emeralds occupy a clear price tier substantially below natural emerald, but generally above flux-grown synthetics, reflecting the more difficult growth process and the better optical properties. Stones marketed as Linde during the production era and into the 1980s now circulate primarily through secondary markets, often unattributed to the original producer. Modern hydrothermal emeralds from Russian and Chinese growers occupy a similar gemmological space.