Grape agate

Grape agate typically occurs as clusters of spherical crystal growths, some weighing up to 80 kilograms (180 lb).^[2][3] Single and double spheres can also be found.^[2] The average diameter of each individual sphere is 3–6 millimetres (0.12–0.24 in), rarely reaching up to 18 millimetres (0.71 in).^[2][3] Many are coated in druzy quartz, giving them a sparkling appearance, while others are smooth.^[3][5] The vast majority of grape agate found is dull-colored, with only about 10% occurring in attractive colors. Of this, approximately 80% is purple, 15% is green or blue-green, and 5% is blue, pink, or white.^[2][3] Multiple colors may occur together in the same cluster.^[1]

The first pieces of grape agate were discovered in 2014 on Sulawesi Island, Indonesia.^[2] They were initially found on a beach at Pantai Ngalo, some 25 kilometres (16 miles) south of Mamuju city.^[3] The name batu Manakarra ("Manakarra stone") refers to Manakarra beach,^[2][3] a well-known beach in Mamuju. This name may have been chosen by an early dealer as an attempt to conceal the true origin of the stone from potential competitors.^[3]

This initial material was fragile and mostly dull brown to gray in color, making it largely unsuitable for collectors and lapidary use.^[2][3] Miners began following streams from the beach up into 500 metres (1,600 ft) tall hills and valleys, finding additional deposits along the streams.^[3] By 2016,^[2] less weathered, shinier, purple and blue-green stones had been found in Takandeang, Tapalang region, Mamuju district.^[3] Miners dug deep trenches and open pits into the jungle hillsides to extract the stones.^[2][3] Subsequently, this higher-quality material began to be sold as collector's specimens and lapidary material at rock shops, mineral shows, and online marketplaces.^[2][5]

Grape agate is sourced from pockets of blue-green clay found between weathered pillow lavas.^[2][3] These lavas are porphyries^[3] of Miocene age^[2][3] and believed to have an intermediate composition of trachyte and andesite.^[3] They are associated with mafic volcanic rocks including basalt, breccias, and tuffs.^[2][3]

Pillow lavas form when lava cools rapidly from underwater fissures. According to geologist Joel Ivey, this likely formed a layer of volcanic glass, which is high in silica, around the lava pillows. Hydrothermal fluids dissolved silica from this glass and gradually decomposed the lava into a magnesium- and calcium-rich clay, filling the spaces between the pillows. Eventually, silica precipitated out of solution to form quartz spherulites within the clay, which settled and bonded together to form clusters. Some individual spheres can also be found on top of the clay.^[3]

Grape agate is a botryoidal form of quartz (silica, SiO₂) and may be considered amethyst when purple in color.^[2][3][5] Like other amethyst,^[6] the purple crystals were determined by X-ray spectroscopy to contain trace amounts of iron (about 150 ppm) in addition to smaller traces of zinc and gallium.^[2][3] The purple color is caused by radiation from decaying uranium in the surrounding rock which activates color centers around the iron impurities.^[2][6] The blue-green crystals, which are likely colored by inclusions of clay minerals, were determined to contain very high levels of magnesium (almost 1%) and calcium (0.2%) as well as traces of iron (1000 ppm) and minor traces of several other metals.^[2][3]

Mineralogically speaking, grape agate is not an agate, since it lacks banding.^[1][7] While commonly referred to as chalcedony,^[1][8] macroscopic drusy crystals on its surfaces and a course texture indicate a macrocrystalline structure instead of the sub-microscopic or cryptocrystalline structure of chalcedony.^[2][3][5][9] Each grape agate sphere is a radial spherulite structure, an aggregate^[5][1] of small, fibrous quartz crystals twisted along their c-axis (lengthwise).^[2][3]

Colorful pieces of grape agate are sought after by mineral collectors and used by lapidary artists as a gemstone.^[2][3] Roughly 40% of the material with desirable colors is suitable for lapidary purposes; the rest is sold as specimens. Duller individual spheres can be tumble-polished,^[2] and some dull-colored grape agate may be dyed a darker purple.^[1] Single spheres may be drilled and used as beads, while small clusters may be wire-wrapped or cut into cabochons for use in jewelry.^[2]