Something unknown and mysterious, referred to in secret passphrases of unclear origins; most likely some specific mineral or stone.
Hubert turned slowly:
—That's right, and that's the way Shaab wishes it.
—Until the Timestop comes.
—Until the agefulness of the Shellrocks takes place and until the Markword starts to shine.
—The Omnibook shall be the Sign to you.— finished Doctor Barton the exchange of conspiratorial rejoinders. (Source: J∙ Okram - The Mystery of the Rammed Key.)