They are precious and semiprecious stones that are wrapped with threads. They "conduct" the relationship between people. They are usually twin stones that glow when the holder of the other is in danger. They only work for the relationship that they are made for, including the people in that relationship.
Threadstones are made by using physical threads as a conduit for actual Threads that bind people. They are then fastened around the stone using the Arlenni Loop, and Ancient Threadwitch technique, most famously adopted by the Sightwitch Eridysi in her creation of a portal.