# Trapiche emerald

- Page: https://taslar.bumba.tr/en/stones/trapiche-emerald/
- Turkish: Trapiche zümrüt
- Kind: Variety
- Classification: Minerals › Silicates › Cyclosilicates
- Chemical formula: Be₃Al₂Si₆O₁₈
- Crystal system: Hexagonal
- Hardness (Mohs): 7.5–8
- Specific gravity: 2.63–2.97 g/cm³
- Refractive index: 1.565–1.61
- Lustre: Vitreous (glassy), Resinous
- Transparency: Transparent, Translucent
- Colours: green, deep green, pale green, black (arms), whitish (fine-grained beryl)
- Streak: White
- Cleavage: Imperfect on {0001}
- Fracture: Conchoidal to uneven
- Value ladder: 7/9 step, Precious stone

A rare emerald, almost only from Colombia, whose cross-section shows green wedges split by six black arms, like the wheel of a sugar-cane mill.

A trapiche emerald is an emerald that, when cut across its long axis, shows a core with six dark arms radiating from it. The arms are made of black shale (a dark, fine-grained sedimentary rock) trapped as the crystal grew, and wedges of green emerald lie between them. The name comes from “trapiche”, the Spanish word for the cogwheels of the mills used to crush sugar cane. It is very rare: almost all known examples come from the Muzo, Coscuez and Peñas Blancas mines in Colombia’s western emerald belt. It is a natural curiosity prized by collectors.

## How does it form?

Colombian emeralds formed from hot, very salty waters circulating through fractures in layers of black shale. These were basin waters that had reacted with salt deposits left by evaporation, and, judging from droplets of fluid trapped inside the crystals, they were at about 300–330 °C. The emeralds of the western belt are 38 to 32 million years old, according to mica minerals in the same veins.

According to a study published in 2015, a trapiche emerald forms like this: fluids build up at the tips of faults until the pressure is so high that the rock suddenly cracks and the pressure drops at once. This sudden drop leaves the fluid supersaturated, and emerald seeds start to grow where the fluid touches the black shale. As the crystal grows along its six side faces it pushes the shale material in front of it aside; that material gathers at the corners, forming the six dark arms and the fine branches between them. After the pressure drop, a regular layer of gem-quality emerald sometimes grows around it all. There is still no full agreement on what causes the pattern.

## What is it used for?

- **Collecting:** with patterns that differ from stone to stone, trapiche emeralds are mineral curiosities sought by collectors; rough crystals and slices are kept in collections.
- **Jewellery:** cut as cabochons or slices to show the pattern at its best, they are set in rings, pendants and earrings.
- **Science:** they are studied to understand how crystals grow, how the surrounding rock affects that growth, and how Colombia’s emerald deposits formed.

## Other trapiche gems

The “trapiche” pattern is not unique to emerald. Rubies from Myanmar, tourmalines from Zambia, chiastolite (a variety of andalusite) and some garnets also show boundaries of inclusions separating equivalent growth zones of the crystal in a similar way. The number of arms depends on the crystal’s symmetry: hexagonal emerald has six arms, while trigonal tourmaline has a three-sectored core. Very few trapiche emeralds have been reported from outside Colombia: one crystal each from the state of Goiás in Brazil and from the Mananjary area of Madagascar.

## How to recognise it

- Its hallmark is the six-armed, wheel- or cog-like pattern seen in a section across the crystal’s long axis: a hexagonal core in the middle (sometimes just a point), six dark arms running from the core to the edges, and green emerald wedges between them.
- The dark arms are made of inclusions such as carbon-rich black shale, quartz and albite; in some specimens the arms are whitish.
- In jewellery it is usually cut as a cabochon (a dome) to show off the pattern; collectors also keep rough crystals and slices.
- It is as hard as emerald (Mohs 7.5–8) and scratches glass.

## Fakes and imitations

- Other stones with a trapiche pattern: trapiche rubies from Mong Hsu in Myanmar and trapiche tourmalines from Zambia also show arm patterns. Tourmaline has a three-sectored core; a laboratory settles what the stone is.

## What determines its value?

The sharpness and regularity of the pattern, the symmetry of the six arms, the colour and clarity of the green wedges and the size set the value of a trapiche emerald. Because it is so rare and every pattern is different, its value rests more on collector interest than on the usual emerald grading. Colombian origin and the fact that the stone is natural should be backed by a laboratory report, and treatments common in emeralds, such as filling fractures, should be disclosed.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

## Myths and misconceptions

- Myth: Trapiche emeralds come from Colombia’s Chivor mine. Fact: An influential study published in 1970 stated that the thousand or so trapiche emeralds it examined came from Chivor; in 1985 it was made clear that the samples actually came from the Peñas Blancas mine. The mix-up is thought to stem from Peñas Blancas samples being passed on by the manager of the Chivor mine. In Colombia, trapiche emeralds occur only in the black shales of the western emerald belt.
- Myth: A trapiche emerald forms when several crystals are twinned and fused together. Fact: Some researchers thought so in the early 20th century, but optical studies showed that these crystals are not twinned. According to current explanations, the pattern forms because a single crystal grows at different rates in different directions and, as it grows, pushes the surrounding black shale aside and traps it between the arms.

## Did you know?

- The first known description of trapiche emerald was given by the French mineralogist Émile Bertrand in 1879, at a meeting of the Geological Society of France, where he presented 40 “curious emerald crystals” from Muzo.
- The name “trapiche” was coined in 1964 by a researcher named McKague; the pattern reminded him of the cane-crushing gears used by farmers.
- A famous trapiche emerald known as the “Star of the Andes” weighs 80.61 carats.
- In 1948 a researcher named Barriga Villalba described Muzo’s trapiche emeralds as of scientific interest but no gemmological value; today they are among the emeralds collectors seek most.

## Where is it found?

- Muzo, Boyacá, Colombia (Type locality): In 1879 the French mineralogist Émile Bertrand presented 40 “curious emerald crystals” from Muzo; this is the first known description of trapiche emerald.
- San Pablo de Borbur (Coscuez and Peñas Blancas mines), Boyacá, Colombia (Major deposit): Coscuez lies about 10 km north of Muzo. New trapiche emeralds were found at Peñas Blancas in 1963; between 1963 and 1970 trapiche emeralds came only from the Peñas Blancas and Muzo mines.
- Goiás, Brazil (Known locality): One of the very rare trapiche emerald crystals reported outside Colombia (1994); the source names only the state, and the point is the centre of the state.
- Mananjary area, Madagascar (Known locality): A single crystal with the spoke-like pattern of Colombian trapiches was reported (1998).

## Sources

- [Colombian Trapiche Emeralds: Recent Advances in Understanding Their Formation (I. Pignatelli et al.), Gems & Gemology 51(3)](https://www.gia.edu/jp/gems-gemology/fall-2015-colombian-trapiche-emeralds-recent-advances-understanding-formation), GIA, 2015
- [Trapiche emerald](https://en.wikipedia.org/wiki/Trapiche_emerald), Wikipedia
- [Coscuez Mine, San Pablo de Borbur, Boyacá, Colombia](https://www.gemdat.org/loc-743.html), GemDat
- [San Pablo de Borbur](https://en.wikipedia.org/wiki/San_Pablo_de_Borbur), Wikipedia
- [Muzo](https://en.wikipedia.org/wiki/Muzo), Wikipedia
- [Goiás](https://en.wikipedia.org/wiki/Goi%C3%A1s), Wikipedia
- [Mananjary](https://en.wikipedia.org/wiki/Mananjary), Wikipedia


Source: https://taslar.bumba.tr/en/stones/trapiche-emerald/ · Licence: CC BY-SA 4.0