I visited one of the islands where those stones were mined. The value wasn't much so in the rock itself, but what someone had to do to produce one. You had people on the island of Yap go by a boat made from a couple of palm trees travel to Palau to quarry the stones, cut them and then carry back. If you had a rai stone, it meant that you were an organized, and formidable navigator.
They navigated some 1500 miles over open ocean. They could detect subtle ripples in the ocean created by waves going around an island, and follow them to get to shore from 100 miles off in open ocean. Being able to produce a stone like was something like bringing back moon rocks from another planet, that weighed a lot. It was the size of the stones that may hauling it back in an out-rigger boat so impressive. This is the kind of boats they used: