In Indonesia, there are thousands of coal deposits. There is a great range of maturities of coal in different deposits from low grade/low calorie lignite to high grade/high calorie anthracite. A year 2000 estimate by the Indonesian Department of Energy and Mineral Resources stated Sumatra had reserves of approximately 17.8 billion metric tons of coal. Extensive coal should relate to an abundance of Amber but much of the Amber which I had previously seen, was black or brown and muddy looking.
These dull dark copals and Ambers have long been produced as a bi-product of coal mining and were routinely exported to China by the container load. I assume they were used to make varnishes or incense.
Several mining companies are exploiting high grade anthracite coal in Sumatra. The ages of these deposits are between 23 and 50 million years old. I have been driving past their operations for 26 years on the prowl for agates, jaspers and other lapidary materials or conducting geological and mineral surveys.
Reports of higher grade coal situated within lower grade deposits suggest they were upgraded by heat from igneous intrusions from nearby volcanic activity. On occasion I found petrified woods near these coal deposits.
If you look at a map of the western margin of the island of Sumatra, the topography is speckled with volcanoes, many of which are still active and periodically erupt. The Island’s western edge is a plate subduction zone and hub of volcanic and seismic activity. Great for gem grade amber production, like an oven with the cook routinely shaking the pot! 5 years ago, my Indonesian rock hounding buddies started to flock to Sumatra following rumors of a gemmy Blue Amber.
This gemmy Blue Amber comes from the east flank of the Bukit Barisan mountain range. Specifically, in the northwest corner of the South Sumatra Basin, a massive area of coal deposits about 150 meters above sea level. The amber has been studied by the GIA (USA), and the GIT lab in Thailand (articles: THE COLOR PHENOMENA OF BLUE AMBER – Gems & Gemology, Summer 2014, Vol. 50, No. 2 and THE CHARACTERISTICS OF BLUE AMBER FROM INDONESIA
Geologically, this deposit occurs in the Sinamar Formation which is an early Oligocene age rock formation (approximately 23 million years old). These rocks have locally been intruded by younger volcanic dikes and sills. Subsequently the coal and amber were subjected to volcanic heat while buried deep in the sedimentary basin. Just one of 2 dozen active modern day volcanoes on the island of Sumatra, the Mt. Kerinci volcano lies just to the west and is a reminder of how natures kitchens work. Recent uplift has