Though made of cut glass, a weirdstone appeared to be a faceted diamond about the size of a humanoid's clenched fist. When active, it pulsed with white light.
When placed on a solid, flat surface, the weirdstone would float three feet into the air and began glowing. While in such a state, it rendered all divination magic unable to detect anything within a six mile radius (which included six miles above and beneath it). Extraplanar movement, movement via plants with druidic magic and the natural abilities of some creatures such as xorns, would not function within that same area. All such abilities were disabled, whether magical, supernatural or psionic in nature.
Deactivating a weirdstone was as simple as activating it - one merely needed to snatch the item from out of the air and place it inside a container or onto a less regular surface, just as long as it wasn't flat enough to activate it again.
Only the creature that activated it in the first place could deactivate it in this way. Others had to smash it, though being made out of glass, this was not a particularly difficult task.