Rabanus, a follower of Alcuin, born in 776, was the author of an
interesting encyclopædia, rejoicing in the comprehensive title,
"On the Universe." This work is in twenty-two books, which are
supposed to cover all possible subjects upon which a reader might
be curious.... The seventeenth book is on "the dust and soil of
the earth," under which uninviting head he includes all kinds of
stones, common and precious; salt, flint, sand, lime, jet, asbestos,
and the Persian moonstone, of whose brightness he claims that it
"waxes and wanes with the moon." Later he devotes some space to
pearls, crystals, and glass. Metals follow, and marbles and _ivory_,
though why the latter should be classed among minerals we shall
never understand.

[Illustration: IVORY TABERNACLE, RAVENNA]

The Roman diptychs were often used as after-dinner gifts to
distinguished guests. They were presented on various occasions.
In the Epistles of Symmachus, the writer says: "To my Lord and
Prince I sent a diptych edged with gold. I presented other friends
also with these ivory note books."

While elephant's tusks provided ivory for the southern races, the
more northern peoples used the walrus and narwhale tusks. In Germany
this was often the case. The fabulous unicorn's horn, which is so
often alluded to in early literature, was undoubtedly from the
narwhale, although its possessor always supposed that he had secured
the more remarkable horn which was said to decorate the unicorn.

Triptychs followed diptychs in natural sequence. These, in the Middle