In the intervening years it has been revealed that the source of "root amber," as exploited by Chinese manufacturers, is BORNEO (!).

On my two trips to Borneo (2010 and 2015) I had hoped to consult with anyone who might know more about this amber (geologically and historically), but I was not able to find anyone who was conversant.

Also, about five years ago, a new source of SE Asian amber has been discovered and exploited. This is located in Sumatra--and the material has much in-common with Borneo root amber; including material that is a combination of dark translucent resin and pale opaque resin. They are calling their material "zebra amber," because it tends to be layered and present "stripes"--rather than being blotchy (like Chinese/Borneo root amber).