Magical items - Twisted Black Amulets

Recent advances in demonology have provided us, as you well know, with a whole set of new spells. One of these is Professor Flambardie's Grim Amulet, which creates a magical amulet that protects from some of the worst aspects of devil-dabbling-in, and which is of obligatory use for two of the evil imp spells: Kelleflump's Irritating Demon and Doctor Kelleflump's Deadly Demon.

The amulet is a curious work of art, certainly worth noting:

This piece of black-stained silver isn't quite flat, and isn't evenly rounded, as though it had been warped by a fire. The spikes and hooks around the rim have a disturbingly organic look, and the eldritch runes spiralling around the face are never the same twice. Occasionally, you can make out glimmers of meaning in the runes, but the bitter, doleful words always retreat and change before your mind can do more than take a glimpse of the vast, empty infinity of unpleasant truth behind the blackened silver.
Unpleasant-looking spikes and hooks of octarine light thrust at odd angles from its surface.
It is in excellent condition.

While being used the amulet doesn't seem to accumulate any visible damage. It will, though, from time to time, cause a small amount of damage to its user ("Your twisted black amulet emits a sullen black gloom and the surface shines with frost"). And after some more time and use, it will melt away. Many hypotheses have been suggested about how and why this happens: some think alignment might have something to do with it, that the number of thaums present when the spell to create it was cast could be decisive, or that the success in the skills when casting the spell is what really cuts the cake... Nothing is known for sure. It does seem to behave like an item with a fixed number of charges, variable in each amulet (like the pickling stick, so maybe also influenced by casting skills), but it doesn't respond to investigations with Fabrication Classification Identification ("The weave abruptly writhes, throwing out nasty-looking spikes and hooks. A black stain spreads over its surface before it shifts... sideways... and disappears somewhere to the left of down.").

Some research lies, therefore ahead. Meanwhile, we'll continue with the cumbersome crafting of these strange artefacts, that show an annoying tendency to unharness all bounds and set free ugly oozing demons into the Discworld...