It' s true to say that they are "fossil wood", "fossil palm wood", "opalised wood" and "petrified wood".
Opalizing palm tree or any other type of timber is fossilization (although different from agatizing). Part of the reason for this is that not all Pumtek pearls are made of palm-tree. A friendly mineralogen recommended that other types of fossile woods be used in the production of some Pumtek pearls.
He was the first to announce that the opalised fossile Palm tree timber (as defined by the Smithsonian). But even he was not sure, because he is not a Mineralogue and knew opals only as the colourful gemstone most would think of - that Pumtek pearls are definitely NOT made.
But when I realised that Pumtek pearls were far too heavy to be this kind of thing, I realised that it was an organically derived one. So fossilized Palmtree made perfect sense. So. It originates from the West Burma area ( "near India") - where a lot of fossile Palmtree is definitely available.
However, when palms have been petrified and turned into opals, it could also transform some of the area' s growing vegetation. Even if the overwhelming part of Pumtek pearls are made of opalised Palmtree heartwood, this does not mean that there are no exemptions. Some parts of the forest may have had bigger and more exposed voids than other parts of the same.
Or maybe younger plants had smaller pores/grains. Maybe a part of the timber rotten and loses its exact texture before opalescence. A number of factors explain why some pearls show the classical veining of fossil/opalized tree ligneous woods, others do not.
Allen Jamey "Burmese pearls comprise Pumtek's (meaning "buried thunderbolt") and Pyu pearls (800 BC-200 AD), as well as some other modern objects.