Sanctum Snail

The legendary sanctum snail was hunted to near-extinction thanks to the unique properties of its shell. A unique form of giant snail, the top of its shell stood four feet above the ground, and the shell itself was as strong as adamantine while only one-third the weight.
The snail was attracted to magical dweomers in areas of high magic, and, like the caterpillars that produce mage-silk, it gained special properties from the magically-infused plant matter it ate. A dedicated herbivore, it could eat a tenth of its body weight in a single day.
If disturbed or frightened, the sanctum snail would retreat into its impenetrable shell and would remain protected possibly for days, until it was certain the threat was gone. Thanks to the properties of its shell, however, hunters were more than willing to gradually creep up on a snail and take it unawares. After surprising the snail they would tip it on its side and attack the vulnerable underside.
Once a sanctum shell was obtained, a multi-stage alchemical process was required to first make the shell usable, then re-harden it in its new form. During the preparation, special dyes, fixatives and polishes could be added to the shell to permanently change its colour. The result left the shell with an iridescent surface resembling mother-of-pearl with a wash of colour tinting it, and is extremely attractive.
Dyed snail shell had a wide variety of uses and useful properties, ranging from hair-combs to jewellery and lightweight yet strong ornate armour. Noblewomen and female assassins particularly favoured hair accessories that were dyed to match their outfits, attractive enough to be worn alongside the finest silks and gems, yet strong enough to have part of it shaped into a concealed weapon without adding to its weight. Such jewellery is still highly desirable in noble houses, and a single piece would be regarded as a princely wedding gift.
Wealthy nobles were particularly fond of the capability of the sanctum snail's shell to be formed into suits of ornate parade armour which were lightweight enough to not inconvenience them whilst retaining the strength and protective qualities of armour thrice as heavy. These suits became rarer and rarer as the snails died out, some armour disappearing into hoards or vaults, others damaged beyond repair or lost at sea or in subterranean ventures. The once-high cost of a full suit of sanctum shell armour is today insanely exorbitant, affordable only by heads of state and those who carefully maintain ancient suits of heirloom armour which have belonged to their families for centuries.