Did Gandalf make a mistake at Isengard ?

[William Cloud Hicklin]

So, in Tolkien's own words, only Saruman had much interest in, or knowledge of, the Stones and he never mentioned them to other members of the White Council. In other words: Gandalf probably did not know about the Orthanc-stone.

I would also mention Denethor's words, "Though the Stones be lost, they say." It's of course a freighted line, because Denethor knows they're not lost, and Gandalf knows they're not lost, and they each don't know, even if they may dimly suspect, that the other knows the truth. Nonetheless: "They say." In other words, the Conventional Wisdom was that the Palantiri had all disappeared.