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Gem Materials of Mexico and Guatemala

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 Queretaro, located 215 kms NW 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

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