They were very heavy but perfectly smooth, and would suffer no damage if by accident or malice they were unseated and rolled off their tables. They were indeed unbreakable by any violence then controlled by men, though some believed that great heat, such as that of Orodruin, might shatter them, and surmised that this had been the fate of the Ithil-stone in the fall of Barad-dur.

Though without any external markings of any kind they had permanent

The Unfinished Tales, page 409

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