Quartz

specimen number:
2292169

location:
Mamaju, Sulawesi, Indonesia

description:
Attractive lustrous purple botryoidal crystallized quartz. Complete all around with no points of attachment and in very good condition with just slight damage present. This is sometimes referred to by the marketing term grape agate, despite the fact this is not an agate.

The Mamuju locality, situated within the Mamuju Regency of West Sulawesi, Indonesia, is a notable mineral-producing area that gained widespread international attention in the mid-2010s. Geologically, the region is dominated by Miocene to Pliocene volcanic and plutonic rock sequences, particularly alkaline basalts, dacites, and andesitic tuffs that underwent extensive hydrothermal alteration. Small-scale artisanal mining and surface quarrying operations began intensely around 2015 following the discovery of unique crystal pockets within weathered volcanic beds. These informal, non-mechanized mining efforts peaked between 2016 and 2020, with local diggers excavating shallow trenches and hillside pits to extract specimen-grade material, and localized hand-mining continues sporadically to supply collectors into the 2020s.

The highly sought-after material found at Mamuju is a unique variety of quartz commonly marketed under the trade name grape agate, though it is not actually a true chalcedony agate. Geologically, this material consists of botryoidal aggregates of tiny, distinct macrocrystalline amethyst crystals that grew radially from central nuclei within the cavities of the altered volcanic host rock. Unlike true agate, which is a microcrystalline variety of quartz featuring parallel bands of chalcedony deposited in sequential layers, the Mamuju quartz is composed of minute, faceted crystal terminations that give the grape-like purple spheres a distinctive sparkle or druzy luster.

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