Tolkien fans, assemble!
There is an ambiguous line in his 1933 poem Errantry:
Of crystal was his habergeon
I’m not sure what a habergeon is, but the man is wearing one.
Thank you, FSU!
All right, since this poem is silly fantasy and not something like The Lord of the Rings, let’s just imagine each of those leaves is made, not of mail or mithril, but of crystal.
What sort of crystal is it? Probably something sparkly but tough enough to deflect weapon thrusts without shattering.
Tolkien has already mentioned diamond earlier in the poem, so perhaps he means one of the clear gem forms of quartz – a hard mineral that can substitute for diamond and comes in six-sided crystals.
So…is the “merry passenger, a messenger, a mariner” wearing colorless rock crystal or one of colored quartzes, say, smoky brown, purple (amethyst), or yellow/orange (citrine)?
Let’s look at the colorful quartz first.
Smoky quartz
Quartz is silicon dioxide, one of the most common minerals in Earth’s crust. Its crystals generally can be either microscopic or, like those we’re looking at today, visible with the naked eye.
It will turn beautiful shades of brown, gray, or even black when it is irradiated, either naturally or by man, without picking up any radioactivity.
In some languages, including the German Tolkien knew and loved, this smoky quartz is called “morion,” a misreading of Pliny’s “mormorion.” That, of course, brings the word “Moria” to mind, but I have seen no mention of a connection in the author’s mind between the dwarf mines and smoky quartz.
The Cairngorm variety of smoky quartz is used as jewelry and as decoration on the hilts of knives. How would it look on a habergeon?
Amethyst
Experts say that, if this gem variety of purple quartz were rare, it would be far more expensive.
Amethyst is the traditional February birthstone. Its unique purple shades come from iron that mixed in while the crystal was forming.
Amethyst has variations in color, shape, and material that uniquely identify crystals according to locality.
Down through history, people have taken advantage of this gem’s hardness to shape it into such things as drinking vessels (“amethyst” comes from the Greek for “not drunk”) or beads. Amethyst carvings are intricate works of art.
How would this look on our merry messenger’s habergeon?
Citrine
No one is quite sure how quartz gets this lovely yellow color.
Citrine crystals are quite rare in nature, and some show smoky inclusions called phantoms. This raises the suspicion that radiation turns quartz yellow as well as smoky brown to black.
Synthetic citrine crystals have been grown using iron (I don’t know why the iron doesn’t turn them purple, as it does amethyst). However, these crystals have slightly different properties from natural citrine.
Most citrine gemstones are actually heat-treated amethyst or smoky quartz. They tend to have more a reddish or orange color than the natural stone.
Citrine has an undeserved reputation as a “fake” thanks to unscrupulous dealers who sometimes try to pass it off as the much more expensive topaz. It really is a beautiful gem in its own right. Perhaps that’s why we have made it, rather than the traditional topaz, the modern birthstone for November.
How would it look on our hero’s habergeon?
Rock crystal
This clear, colorless gem variety of quartz, the traditional April birthstone, is what inspired the English word “crystal.”
Back in the days of Pliny, rock crystal was thought to be water ice that had somehow fossilized.
Of especial note is that in Old English (Tolkien’s specialty), rock crystal was “cristal.”
Tolkien was also fascinated by the Franks. Lothair, King of the Franks, had a rock crystal gem carved in the 9th century.
Rock crystal vases and other items were also treasured during medieval times.
Yes, this is also the stone used in crystal balls and skulls.
Overall, I think it likely that Tolkien meant rock crystal when he described his mariner in Errantry.
Of course we will never know for sure what crystal J. R. R. Tolkien had in mind when he wrote Errantry.
It was probably quartz. I believe it was clear rock crystal, but it’s fun to read through the short poem and imagine its hero stalking around in smoky quartz, striding along imperially in purple amethyst, or battling the “Dumbledores (no, Rowling did not invent the name), the Bumbles, and the Honeybees” in glistening yellow citrine.
Tolkien did tell us his hero’s sword scabbard was made with chalcedony, a member of the quartz family that has microscopic crystals. We’ll take a look at that next week.
Part 3 of this four-part series will look at malachite, and part 4 will examine the biological treasures – coral and ivory – mentioned in Errantry.
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More information about quartz crystals:
Flight To Wonder 
Great readding