Fossil records show that the foraminifera used tiny diamond particles, created by the giant meteor impact that killed the dinosaurs, as a protective coating. Said the New Scientist:
It seems the main reason the foraminifera chose grains containing diamonds and metals for their tests was the lack of other grains of a similar density at this particular site. What remains a mystery is how these ancient foraminifera were able to pick out the densest grains, an ability they share with their latter-day descendants.
Or maybe, just maybe, they liked to do it up big style.