The Gachala emerald mining district lies in the eastern emerald belt of Colombia in the Cundinamarca department, approximately 80 miles northeast of Bogota. Together with Muzo and Chivor, Gachala is one of the three classical sources of fine Colombian emerald, and the district has produced commercially since at least the 1940s. Gachala material is sometimes characterised in the trade as having a slightly bluer cast than Muzo or Chivor, although the visual differences are subtle and not reliable on their own as origin indicators.

Geology

The Gachala mines work hydrothermal emerald deposits hosted in carbonaceous black shales of Cretaceous age. The host rock is the Macanal Formation, a sequence of dark shales with interbedded limestones, deposited in a subsiding rift basin during the Lower Cretaceous (approximately 130 to 100 million years ago). The mineralisation is associated with calcite and pyrite veins that cut the shale, and emerald crystals occur in cavities and fractures within the veins along with calcite, dolomite, fluorite, parisite, and minor quartz.

The geological model for Colombian emerald formation, developed primarily by Cheilletz and others through the 1990s, identifies the source of the chromium and vanadium colour-causing elements as the host black shales themselves, leached by hot saline brines that circulated through the sedimentary sequence. The beryllium required for emerald formation has a less clear source but appears to have been mobilised from the same sedimentary system. This sedimentary geology is unusual; most emerald deposits worldwide form in metamorphic or pegmatitic settings where granitic fluids encounter mafic or ultramafic rocks containing chromium.

Production history

Commercial production at Gachala accelerated from the 1940s onward, although small-scale working in the area predates this period. The principal mines in the district include Vega de San Juan, where the 858-carat Gachala Emerald was found in 1967 (now in the Smithsonian); La Vega, El Diamante, and several smaller workings. Production through the 1970s and 1980s was substantial but variable, and the district was affected by the violence associated with the Colombian emerald wars of that period, in which competing emerald-trading factions in Boyaca and Cundinamarca engaged in extended armed conflict that affected mine security and production.

Following the resolution of the principal conflict in the early 1990s through agreements brokered by the Catholic Church and by the cuartel emeraldero (emerald boss) Victor Carranza, Gachala continued as a producing district at lower volume than the larger Muzo operations. Modern production at Gachala is generally smaller in scale than at Muzo or at the recently renewed Chivor operations.

Material character

Gachala emeralds are characterised by the gemological features common to Colombian emerald: hydrothermal three-phase inclusions (gas, liquid, and a small cubic crystal of halite, in the same cavity), parisite needles, and irregular jardin inclusions. The colour ranges through medium to medium-dark green, with hue tending slightly toward bluish-green compared to the slightly yellow-green tendency of Muzo material. The distinction is subtle and individual stones from any of the three districts can fall outside the typical range. Trade attribution by visual character alone is unreliable, and origin determination by gemological laboratory uses trace-element chemistry and inclusion patterns rather than colour alone.

The clarity of Gachala material varies as widely as it does at the other Colombian sources. The visible inclusions in Colombian emerald are accepted as part of the character of the stone, and the trade does not penalise jardin to the same extent it would on, for example, Brazilian or Zambian emerald. Most Colombian emerald is fissure-treated with cedar oil or with proprietary resin formulations to improve apparent clarity. The treatment is disclosed by reputable laboratories on origin reports and by graduated terms (none, minor, moderate, significant) on treatment reports.

Position in the contemporary trade

Gachala is a smaller district than Muzo in current production, but Gachala material continues to enter the trade through both formal and informal channels. The mines and the trading houses in Bogota's Avenida Jimenez emerald district handle the bulk of legitimate Colombian emerald trade, with the rest cut and traded primarily in Jaipur, Bangkok, Hong Kong, and New York. The premium that fine Colombian emerald commands over equivalent material from Zambia or Brazil reflects both the visual character and the historical resonance of the Colombian sources, of which Gachala is one.