Grape agate is the name given to a purple, botryoidal form of chalcedony found in Manakarra on the Sulawesi Island of Indonesia. It forms in clusters that often resemble bunches of small grapes, and colors can range from pale to deep purple to green. Some specimens can be quite sparkly due to minute quartz crystal growing on the spheres. Grape Agate is a relatively new material on the market, first being exported from Indonesia around 2015.
Grape agate ranges in color from deep purple to a pale, whitish purple. The darker colors are more desirable and fetch higher prices. Some material can be green due to the presence of clay inclusions. Higher quality grape agate can be quite sparkly due to the presence of micro quartz crystals on the surface of the orbs.
Grape agate is collected from a 25 square kilometer area of rugged, jungle covered mountains in the northern end of the Manakarra mining district on the Sulawesi Island of Indonesia. Often Manakarra Beach which is about 10km south of this area gets used as a locality name. It’s been reported that the first finds of grape agate were small pieces of float found on the beach that had washed down nearby rivers from the mountains. Prospectors followed these rivers into the mountains to locate the deposits that are now being mined. It’s also likely local dealers originally provided Manakarra Beach as a locality in order to lead the competition astray.
The grape agates are mined from in pockets of blue-grey clay in a weathering, andesite bedrock. Local miners work by hand, probing the rock with steel rods to search for these pockets. These pockets are then opened up using hand tools and the grape agate clusters carefully collected. Typically the more vibrant/deeper colored clusters are found deeper, in harder rock that has not yet weathered, possibly due to the exposure to weathering leaching minerals from the grape agates. Some great photos of the mining area can be found here and here.
A New Mineral On The Market?
Grape agate from Indonesia first started appearing on the commercial market in 2015. The deposits were likely known for some time before then. Up until 2015 the Indonesian government banned the export of raw minerals to try and keep lapidary jobs in the country.
How Is Grape Agate Price/Valued?
When grape agate first started appearing on the market it was astronomically priced with some specimens prices in the tens of thousands of dollars. With larger quantities being exported it is much more affordable today, but the majority of it that is being exported is fairly low quality material.
There are lots of factors that play into pricing. Deeper purple colorations are more desirable and thus will be priced higher. Larger sphere sizes will increase the price. How is the luster and sparkle of the specimen? Most of the low quality material is fairly dull in color. Of course the size of the specimen will be a primary factor in the price.
How Was Grape Agate Formed?
Grape agate is found in small pockets of blue-green clay within a host andesite pillow lava which formed during the MIocene when lava flowed into cold seawater. Andesite lava behaves differently than basalt in that it is more viscous, so when it cooled many voids remained between the pillows. The grape agate was then formed by some type of of hydrothermal event, where water deposited silica and other minerals within the pockets in the andesite.
"I thought the orbs must have formed on some sort of nucleus but proof of that is not yet evident.
A crossed polar micrograph of a Manakarra thin section cut by Terry Moxon, exhibited a Maltese Cross. According to Terry, this uncommon phenomenon results from twisting of the quartz C-axis during rapid growth of a fibrous chalcedony. This is in contrast to the layered growths of chalcedony which forms by deposition in concentric layers in a void such as with agates.
The chemical analysis and XRD indicate composition of the Manakarra is dominantly chalcedonic quartz (SiO2) with very minor potassium, aluminum, calcium and magnesium which are likely decay products of the original volcanic host rock. The blue-green illite or smectite clay is also typical of the decomposition of a mafic host. In this case, aside from SiO2, only minor Ca/Mg/K/Na/Al was detected (<1%).
The weight percent of trace elements is so low that I cannot comment on which are likely contributors to the rich colors we see in the Manakarra specimens recovered from in situ volcanic host rocks."
Is Fake Grape Agate A Thing?
There seems to be some misinformation that the Indonesian grape agate is fake. It’s an entirely natural geological formation. I’ve heard that low grade material is sometimes dyed a darker color of purple to increase its value, but I’ve never encountered this dyed material.
The talk of fake grape agate probably comes from decorative, polished stone grapes manufactured in China that have been occasionally sold under the name grape agate. But, they look nothing like real grape agate and would be difficult to confuse with a natural material, even by a novice.
Other Names For Grape Agate?
Grape agate is a fairly new trade name for the mineral having been coined around 2016. It’s also frequently known as Indonesian Purple Chalcedony, Batu Manakarra, Manakarra Agate, etc. Despite the common name grape agate it is not technically an agate because it doesn't have banding but is a form a chalcedony.
What Is Grape Agate Used For?
Grape agate is very coveted among mineral collectors, as it's a very aesthetic looking mineral. High quality specimens with good color, luster and large botryoidal formations can be quite valuable. Small clusters or individual spheres are often used in jewelry and it's become popular with the healing crystal community for promoting "profound peace and tranquility"
Has Grape Agate Been Mined Out?
The grape agate is found over a fairly large area, about 25 square kilometers. The mining is difficult, very labor intensive and done by hand meaning only the deposits near the surface are collected. Given the size of the deposits and difficulty in extracting the material it's unlikely that it will be mined our for some time.
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