Gem Materials of Mexico and Guatemala – Mexico and Guatemala

Mexican Fire Opals

Mexican Fire Opals

Opals

Mexico is famous for its "Fire Opal", which is a type of opal rarely found in Australia. It is defined as a transparent-to-translucent stone with an orange-to-red body color which may or may not show a play of color. Also found is "Water Opal" which is a colorless (or very pale yellow or faint pink) transparent variety, often showing a play of iridescent colors. Jewelry shops and street stalls in Mexico are well stocked with local opal and not once did I see any Australian material.

Mexican opal is found as nodules and veins in weathered volcanic rocks rich in silica. Because of its natural shape and transparency it is usually fashioned into high-domed cabochons and is often faceted into attractive gemstones of oval and cushion-shape. The high quality stones are very attractive with their pure orange body color showing a delicate play of rainbow colors when viewed at different angles. Lessor quality stones are more translucent and may have whitish agate-like banding whilst others have matrix attached, all of which makes for an interesting variety in appearance.
The center of opal production and lapidary work is the state and city of

Querétaro is located 215 kilometres northwest of Mexico City. This Spanish colonial town (population 850,000) has a UNESCO heritage listing for its historic center and is worth visiting to admire the beautiful shops, restaurants and churches. There are many little plazas linked by cobblestone pedestrian malls full of craft stalls.

Opal was first discovered in Queretaro state in 1835, near San Juan del Rio and Tequisquiapan, and the most important mines are San Filipe, Cerro Viejo and La Carbonara. A yellow, and red fire opal are found in a trachytic porphyry in Hidalgo state. Opal occurrences are fairly widespread in the northern volcanic regions of Mexico, and some is found in Guatemala and Honduras. Pre-Columbian relics in anthropology museums show that the Olmecs, Aztecs and Mayans used and traded opal, along with jade and other semiprecious stones.
Climate may be a factor causing opal formation in these volcanic rocks. The Mexican altiplano has an arid climate with any rain confined to the summer months of June to October. Continual wetting and drying out of surface outcrops could mobilize the silica during the weathering process to favor opal formation.

Jade

Mexican and Guatemalan jade is the variety jadeite, which is a pyroxene of ideal composition NaAlSi2O6. It was extensively used in Pre-Hispanic times by the Indians for carving into art works e.g. figurines, beads, pendants and mosaic masks, which were owned by priests and royalty. Weapons of war included jade adzes and clubs. The Mayans and Olmecs even used jadeite for tooth inlays. Drilling, sawing and shaping was achieved by abrasion with wood or slate implements loaded with sand, preferably a red garnet sand obtained from the rivers. Very fine examples of ancient jade lapidary work can be seen in the Museums of Anthropology in Mexico City and Jalapa.

When the Spanish arrived in Mexico circa 1520 AD and the Conquistadors took over the country and enslaved the Indians, the sources of this jade were lost, because they were hidden and the workings became overgrown by the jungle. The Spanish wanted gold and silver and were not impressed by the green stones coveted by the Indians.

The Guatemalan occurrences of jade in the Sierra de las Minas mountains were rediscovered in 1975 by the American archaeologist Mary Lou Ridinger who, with her husband Jay, went on and established a mining operation there, and lapidary works, museum and sales outlet (Jades SA) situated in the beautiful tourist town of Antigua Guatemala in the Western Highlands.

Another center of jade production today is San Cristóbal de Las Casas in the southern Mexican state of Chiapas, which borders Guatemala. The "Casa del Jade" is an up-market jewelry shop and museum with some lapidary work done on-site. The rough material is obtained from the mountains around Chalchihuitín, some 70 kms north of San Cristóbal.
The jadeite comes in a wide range of colors, translucency and purity (more so than nephrite jade).

All types have their special lapidary uses. Colors range from white to black, through brown, orange, pink and many shades of green (apple, grass, emerald etc) and "Galactic Gold" having spangles of pyrite. New colors discovered are lavender, mottled white and blue and light yellow. The most valuable is of emerald green color having high transparency (Imperial Jade). The highly transparent material is of small size and is fashioned into cabochons for fine jewelry.
Jadeite has a granular structure of fine crystals and is often mixed with other minerals such as albite and diopside. Low cost jade pendants (US$5) available from street stalls in Mexico seem to be a jade rock, having a mottled green and white appearance, the green jadeite being perhaps 75% of the whole.

The success of the jade industry in Mexico and Guatemala (insignificant overall) is due to their concentration on up-market sales of fine art work which are bought by wealthy collectors from North America and Europe. They make superb replicas of Aztec and Mayan figurines (labeled, this is a replica of a valuable figurine in a specific Museum of Anthropology) and there is no shortage of Mayan glyphs and other ancient carvings to copy. The latter may be etched on large slabs of polished black jade and look magnificent. Mosaic jade death masks are popular and make fine ornaments for one’s home, but their price was beyond my budget (some over US$4000).
To some extent this idea has been followed in New Zealand where nephrite jade is used to replicate Maori meres and tikis etc. It makes one realize that having a huge deposit of nephrite jade, such as at Cowell, South Australia, is not much good without the expertise of marketing it in a form desired by affluent collectors. 

Amber and Sandstone Matrix

Amber and Sandstone Matrix

Amber

Amber is a fossilized resin originating from pine trees. It is found as transparent lumps, of yellow to red colour, in Tertiary Age sediments of shallow water origin. The classic deposits are along the Baltic coastline of eastern Europe where for centuries amber has been recovered from glauconitic sands of Oligocene Age (ca 30 million years). It is used for ornamentation, medicinal purposes, in funeral rites and production of varnishes and succinic acid.
The ancient Mayans of southern Mexico also used and traded amber.

The present day mines are near Simojovel and Totolapa in Chiapas state, some 80 kilometers north of San Cristóbal de Las Casas, not far from the jade deposits. It occurs in a grey, micaceous, sandstone of late Oligocene to early Miocene Age (20 to 30 million years) with a capping of lignite. It originates from the resin of the tree "Guapinol" and is found associated with fossil brachiopods, gastropods and molluscs. It is a mountainous area and the workings simply consist of adits dug by miners (los ambareros) into the hillside.
In San Cristóbal one must visit the Museo del Ambar de Chiapas which is housed in the Ex Convento de la Merced, to see a huge collection of local amber and to learn the history of its exploitation.

The Simojovel area is the most productive of clear yellow to pinkish amber, whilst the Totolapa amber is more reddish with frequent inclusions of insects. Some 75 different species of insects have been identified including ants, butterflies, spiders, mosquitoes and other flies to 1 cm size. Colors vary (like sherry and port wine) from clear yellow, cognac, red, cream and black.
Clear yellow amber with insect inclusions is highly prized for pendants and display specimens. Polished amber pieces are set in silver jewelry. The local silversmithing is excellent with the usual range of pendants, bead necklaces, cabochons in rings and earrings. Large pieces, 6 to 10 cm size are carved into figurines, birds, animals, frogs etc.

Some expensive necklaces have amber combined with either lapis lazuli or jade. Much of the low cost amber for sale on street stalls is pressed amber, made by heating under pressure small, otherwise useless fragments. Amber melts around 220°C and will aggregate into a single lump, complete with added insect inclusions and many air bubbles. Be suspicious if your amber pendant has an imbedded scorpion or peso coin!

Miscellaneous

Much fine lapidary work is done with obsidian. At Teotihuacán archeological site near Mexico City there is an obsidian workshop where for the benefit of tourists the Indians fashion traditional arrow and spear heads for sale. However, some obsidian has a beautiful flow structure which is emphasized when polished into spheres and other ornaments. The varieties known as Sheen Obsidian (yellow sheen) and Rainbow Obsidian (rainbow sheen) are particularly striking.

The silver mining regions that I visited north of Mexico City (Pachuca and Guanajuato to Zacatecas) are a source of many mineral specimens of interest to the collector. The gangue minerals include quartz crystals often with an amethystine base, calcite as platy rosettes (calcita en hojas) and as spiky "dog tooth spar" and massive fluorite. Silver minerals include native silver, argentite, ruby silver, and the common sulfide minerals chalcopyrite, pyrite and galena are plentiful. I was tempted to buy some colorless topaz crystals and rare danburite crystals up to 5 cm size.

Allan Taylor first wrote this article from notes taken on a 3-month visit to the region; 7
Nov 2004 to 7 Feb 2005, the article being first written for the monthly
newsletter of the "Field Geology Club of South Australia", Bulletin Vol 34, #2, March 2005, pp 5-7.

For more information, check out the author’s website

Traveler Article


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