Indonesia is made up of islands, lots of them, in fact, over 17000 islands. The origins of these islands vary from coral atolls, continental masses or slivers, accreted oceanic wedges and volcanoes rising from the sea floor. All told, it's a great place to hunt rocks formed by hot volcanic fluids circulating thru a multitude of differing geology.
Ornamental jaspers, agates and thunder-eggs are common across the country. Indonesians love natural stone with graphic beauty. There is a big following for picture stones across the country. I have seen agates from Timor, Irian Jaya, Kalimantan, Java and Sumatra. I am sure there are many more locations than I’ll ever have time to see.
The stone markets in Surabaya and Jakarta have a constant flow of new materials. Yet of all the Indonesian agates I have seen to date, none exhibit more variety of color and complexity than the Sumatran Agate (Photos #1 & #2).
The first time I noticed these agates was when some villagers in West Java showed me a large bag of heated red and orange banded and fortification rich agate cabs (Photo #3 & #4).
In the pile were a few cabs with rich yellow, pink, purple and red bands and fortifications. These were cut from 100% natural, unheated agates (Photo #5 & #6).
I decided to study up on agate genesis and what rock types they form in so I could go to Sumatra and track down these beauties.
After a review of studies and theories from the USA, Germany, Russia and the good old internet, it became clear the genesis of agates is complicated! The important components are geological formations consisting of rock types in which voids occur and warm, volcano related, silica saturated waters are present to migrate thru the pores and cracks to fill the void. Voids are commonly known to occur in gaseous volcanic rocks (basalts) or solution cavities in calcareous rocks (limestone). That does not mean there are not a myriad of other source rocks..there are!
The origins of these rocks can be terrestrial or submarine. That didn’t help much in narrowing my search so I left the geological maps behind and struck out to hunt down the perfect agate.
Sumatra is the 6th largest island on earth, 1690km long and 400km wide. There are 37 volcanoes considered recent and most are intermittently active. These young volcanoes are scattered from the far north end to southern tip along the Bukit Barisan Mountain range. In addition, the island has a long history of volcanism. Volcanic rocks, intercalated with marine reefal limestone and other sediments date all the way back into the late Jurassic (about 145mya).
By these associations, I conclude that these agates were likely formed in solution cavities in calcareous marine sediments which were exposed to warm silica saturated (hydrothermal) volcanic waters.
The colors of the bands in these agates vary widely. Coloration of agates is caused by metallic ions which are introduced with the influx of amorphous silica. These elements join together, forming bands and fortifications of micro or crypto crystalline layers which plate out on the walls of the cavity. The banding and coloration depends on the solution chemistry, temperature, pressure, and oxidation state. The hydrothermal fluids travelling thru the pores and cracks in the surrounding rock formations can leach metallic ions and other elements and deposit them in the voids and cavities creating an agate.
Iron alone can cause purple, blue, yellow, orange or red bands. Nickel, vanadium or chromium inclusions show as shades of green. Manganese can exhibit pink, violet or black and copper can be red, blue or green. Combinations of minerals effect color as does temperature, pressure and radiation.
A complex fortification agate is likely formed over a great length of geological time experiencing a great variety of conditions.
There are no two agates exactly alike. Each one is a unique creation of nature! Enjoy Photos #9 thru #20 below: