Indonesian Petrified Wood, Jasper and Opal: What the Volcanic Arc Supplies

Short answer: Indonesia sends the stone trade mostly silica: petrified wood from Java that may be agatized (chalcedony) or opalized (common opal), opaque red-brown river jasper, purple botryoidal "grape agate" from West Sulawesi, and small amounts of unusual opal. Much of it owes something to the volcanoes of Sumatra and Java. Indonesian wood is cheap because supply is large; judge it by grain detail, polish and whether it is opal or quartz.

Why does a volcanic archipelago turn out so much silica?

Indonesia is a country of volcanic islands. The USGS publication This Dynamic Earth notes that most of the world's active above-sea volcanoes lie near convergent plate boundaries where subduction is occurring, and Indonesia's string of cones across Sumatra and Java is one of the busiest examples. Geology.com's Indonesia map page lists volcanic activity, earthquakes and tsunamis first among the country's natural hazards, and notes that Kalimantan is the Indonesian part of the island of Borneo, a much older and quieter block of crust than the volcanic islands to its south.

Volcanoes matter to a stone buyer because they bury things fast, and petrified wood starts with rapid burial; the petrified wood guide covers the mineral replacement that follows. The word for a volcanic mudflow comes from this country: the same USGS text explains that lahar is an Indonesian term for volcanic mudflows, mixtures of volcanic debris and water. A lahar that sweeps through a forest is exactly the kind of rapid burial that gives wood a chance to petrify instead of rotting.

The same USGS text describes the other side of the bargain. Some of the best rice-growing regions of Indonesia are in the shadow of active volcanoes, and most of the metallic minerals mined worldwide, including copper, gold, silver, lead and zinc, are associated with magmas in the roots of extinct volcanoes above subduction zones. That is the setting for Indonesia's metal mining, which is a separate story from the decorative stone sold in crystal shops.

Four eruptions that show the scale

The arc's history is written in some of the largest eruptions known, and the deposits they left are part of the landscape that wood, jasper and opal come out of.

The figures come from Oregon State University's Toba page, which compares the Young Toba Tuff with Yellowstone's eruptions, from the USGS account of Tambora and Krakatau in This Dynamic Earth, from Volcano World on Krakatau, which places it in the Sunda Strait between Java and Sumatra, and from Volcano World on Merapi. Since 1927, small eruptions have built a new island in the Krakatau caldera, Anak Krakatau, the "Child of Krakatau."

What does Indonesia actually supply to the crystal trade?

Very little Indonesian material is sold as single crystals. The country's strength is massive and microcrystalline silica, cut into spheres, slabs, bowls, tumbled stones and furniture.

Petrified wood: agatized or opalized?

This is the single most useful distinction for anyone buying Indonesian wood, because the country produces both kinds and dealers rarely say which is which. Geology.com's opal article defines opalized wood as a type of petrified wood composed of opal rather than chalcedony, almost always common opal without play of color, and adds that opaline wood is often taken for chalcedony because many people do not know petrified wood can be opaline. The same page says the blue opal found in Indonesia is usually associated with opalized wood, which is why pale blue to blue-gray streaks inside Indonesian wood are worth a second look.

The International Gem Society gives the collector names for the opal type: opalized wood is also called xylopal or zeasite. Practically, the two kinds behave differently:

• Agatized wood (chalcedony) is close to Mohs 7, heavier in the hand, and takes a hard, glassy polish that resists scratching. Colors run brown, red, yellow and black.
• Opalized wood (common opal) is softer and lighter. Geology.com gives opal a hardness of about 5.5 to 6.0 and a specific gravity of 2.0 to 2.2, varying with its water content, so an opal log feels noticeably light for its size. Expect a waxy to porcelain luster with white, cream, honey or bluish zones, and keep it out of hot sun and away from heaters, since it holds water.

For the general chemistry of how wood turns to stone, see the fossilization guide, and for collecting rules in the United States and how to judge any petrified wood, the petrified wood guide.

River jasper and other opaque silica

Jasper is one of the quieter Indonesian exports. Minerals.net's jasper gemstone page lists Indonesia among the important deposits, alongside India, Russia, Kazakhstan, Egypt, Madagascar and others, and illustrates a reddish-brown and white jasper from Indonesia. "River jasper" in the trade describes water-worn, iron-stained jasper with earthy red, brown and cream patterns; the planned river jasper guide covers it as a stone in its own right. The broader family is in the jasper guide.

Grape agate from Sulawesi

The purple botryoidal chalcedony sold as grape agate is Indonesia's best known specimen material, from the Mamuju area of West Sulawesi; a specimen photographed in the Mineral Matters column is labeled Mamuju area, Sulawesi Barat Province. It is a separate island from the Java wood and jasper trade and sits in a different volcanic setting. The grape agate guide explains why it is chalcedony rather than agate and how to spot dyed or damaged pieces.

Opal: small, dark and unusual

Indonesia is not a major opal producer, but it does have a recognizable type. The International Gem Society describes Indonesian opal as little known, occurring as thin seams in dark rock, much of it water opal resembling material from Mexico. Its most distinctive product is a black opal of reddish flecks of color swimming in a translucent but very dark brown body. Most gems are under 10 carats, and production is very small. Geology.com includes Indonesia among the countries producing precious opal, behind Australia, Ethiopia and Mexico. The opal guide covers how play of color forms.

Which Java volcanoes connect to the stone trade?

Java is crowded with active cones, and two in the west of the island show how ash, acid water and sulfur shape the ground there.

Papandayan, a complex stratovolcano, has, according to Volcano World, very distinctive yellow crater walls colored by sulfur, with fumaroles, solfataras, mud volcanoes and hot springs on a crater floor about 2,000 m high. A sulfur-encrusted fumarole field occupies Kawah Mas, the "Golden Crater." Its first historical eruption, in 1772, collapsed the northeast flank in a debris avalanche that destroyed 40 villages.

Galunggung, on the west side of Java, is described by Volcano World as a stratovolcano whose 1822 eruption (VEI 5) sent pyroclastic flows and mudflows that destroyed 114 villages. During its 1982 eruption two jumbo jets flew into the ash cloud at about 10 km and their engines stalled before the pilots restarted them.

Collapse, ash and mudflow on this scale are the processes that bury forests quickly, the first requirement for any petrified wood.

How is Indonesian stone mined and supplied?

Published, verifiable accounts of how Indonesian wood and jasper are dug are scarce, and a buyer should be wary of romantic origin stories on listings. What can be checked is the price: wood and jasper are sold cheaply and in volume, in sizes from tumbled stones to large spheres, which says the supply is not scarce.

The clearest documented picture of hand mining in Indonesia comes from sulfur. Geology.com's sulfur page illustrates the blue flames of burning sulfur at Kawah Ijen volcano on the island of Java. A field narrative on Oregon State's Volcano World describes the work in 1997: men carried blocks of sulfur up the steep crater wall in two baskets on a pole, and one said his load was about 80 kg (170 pounds). The summit stands at 2,386 m. That is industrial sulfur, but it is the same volcanic system that leaves small sulfur crystals and crusts for collectors.

Metal mining is a separate, much larger industry. Geology.com lists bauxite, tin, nickel, copper, silver and gold among Indonesia's metal resources, and its cassiterite page names Indonesia among the countries where placer deposits of cassiterite, the main tin ore, are worked today. Collector cassiterite and nickel minerals from Indonesia are uncommon on the specimen market compared with the decorative silica.

How do you judge quality and price?

Indonesian material is abundant, so price follows workmanship and pattern more than rarity.

• Grain and structure. In wood, the best pieces show growth rings, rays or bark texture, not just brown color. A sphere cut from a log shows rings as curved bands.
• Material type. Ask whether wood is agatized or opalized. Opalized pieces with blue zones can be attractive, but they need gentler handling.
• Polish. Chalcedony wood should take a mirror polish. Dull, wavy surfaces suggest a soft or porous zone, or rushed finishing.
• Fillers and cracks. Large slabs and table tops are often stabilized. Glossy, flat spots that do not reflect the grain are resin fills and should be disclosed.
• Color. Natural wood and jasper colors are earthy: brown, red, cream, black and gray. Saturated blue, pink or green is dye; see the dyed stones guide.

Shopping the Indonesian stone we stock

When buying, read the listing for the island or region, the form (natural rough or cut), and any mention of dye or stabilization. For wood, look closely at photos for grain. For grape agate, look at the gaps between spheres for even color. Current stock that shows the range:

• Petrified Wood Sphere: an Indonesian sphere at a modest price, the shape that shows how polish and grain work together on agatized wood.
• Petrified Wood: small rough pieces at an entry price, useful for checking weight and luster to tell chalcedony wood from opal wood.
• River Jasper: Indonesian jasper in earthy red-brown tones, a good example of opaque iron-stained silica from the islands.
• Grape Agate: a natural cluster of purple botryoidal chalcedony, the Sulawesi material in its untreated form.

Frequently asked questions

Is Indonesian petrified wood real?

Yes. It is wood replaced by silica, either chalcedony (agatized) or common opal (opalized). The common issues are not fakes but undisclosed resin filling in large slabs and dyed color. Natural tones are browns, reds, creams and grays.

How do you tell opalized wood from agatized wood?

Agatized wood is harder (close to Mohs 7), heavier and takes a glassy polish. Opalized wood is lighter (specific gravity about 2.0 to 2.2), softer and often waxy, with white, honey or bluish zones. Geology.com notes that opalized wood is often mistaken for chalcedony, so ask the seller.

Where does grape agate come from?

From the Mamuju area of West Sulawesi, Indonesia. It is botryoidal purple chalcedony rather than banded agate, and it is sold as natural clusters.

Does Indonesia produce opal?

In small amounts. The International Gem Society describes thin seams in dark rock, water opal similar to Mexican material, and a distinctive black opal with reddish flecks in a dark brown body. Most gems are under 10 carats.

Why is Indonesia so volcanic?

USGS places most of the world's active above-sea volcanoes near convergent plate boundaries where subduction is occurring, and Indonesia's islands sit in that setting. Its eruptions include Tambora in 1815, which USGS calls the most powerful eruption in recorded history, and Krakatau in 1883.

Sources

• Geology.com, Indonesia Map and Satellite Image: https://geology.com/world/indonesia-satellite-image.shtml
• USGS, This Dynamic Earth: Plate tectonics and people: https://pubs.usgs.gov/gip/dynamic/tectonics.html
• Mineral Matters (Steve Sorrell), Grape Agate: https://mineralmatters.substack.com/p/grape-agate
• Oregon State University Volcano World, Toba: https://volcano.oregonstate.edu/toba
• Oregon State University Volcano World, Krakatau: https://volcano.oregonstate.edu/krakatau
• Oregon State University Volcano World, Merapi: https://volcano.oregonstate.edu/merapi
• Oregon State University Volcano World, Papandayan: https://volcano.oregonstate.edu/papandayan
• Oregon State University Volcano World, Galunggung: https://volcano.oregonstate.edu/galunggung
• Oregon State University Volcano World, Hiking Mount Ijen: https://volcano.oregonstate.edu/hiking-mount-ijen
• Geology.com, Opal: https://geology.com/gemstones/opal/
• International Gem Society, Opal Value, Price, and Jewelry Information: https://www.gemsociety.org/article/opal-jewelry-and-gemstone-information/
• Minerals.net, Jasper gemstone: https://www.minerals.net/gemstone/jasper_gemstone.aspx
• Geology.com, Sulfur: https://geology.com/minerals/sulfur.shtml
• Geology.com, Cassiterite: https://geology.com/minerals/cassiterite.shtml