They do not look like any sort of phenolic beads that I've come across, especially in a graduated strand of rounds. But, I've only been looking at phenolic beads for less than 2 years, and only ones from the African trade. Just dipping my toe into phenolic beads made for the costume jewelry biz recently. And making a lot of mistakes...

Some of the other natural materials like tagua nut are pretty dense and may be possible candidates.

Do you have any info about when & where these were made?