Crystalline Formations of Diamondgate

These formations appear to be composed of a mostly transparent material, though vision through thicker sections quickly becomes impossible due to internal facets.

Theories

Unlike true diamond, the material is incredibly resilient, so much so that many scholars speculate that the material is in fact protected magic similar to a Wall of Force. This theory correlates with the strong magical aura given off by the material, and it is largely agreed to be the result of powerful magics in ages past.

Usage

The most obvious way that the inhabitants of Diamondgate have capitalized on the unusual qualities this material exhibits is in their incorporation of large formations in the city walls, including the entry to the city.

The eponymous gate is actually more of a tunnel, winding its way through a massive chunk and 60 feet wide. Offshoot passages were walled off long ago, and though there is much speculation as to where they lead no known attempt to map them has been made; the local government takes a very dim view indeed of the occasional enterprising individuals who attempt to investigate, preferring to leave the thoroughfare undisturbed.

The use of magic attempted within the formation behaves erratically if at all, and sentient beings who pass through it find themselves overcome by an inability to speak untruths or act against others, sometimes for days. As such a cottage industry has risen up around caring for new arrivals to the city, usually taking up lodgings around the gate until they are able to properly defend themselves against those who might seek to take advantage of them in this state.

Given the obvious defensive benefits that such a point of ingress provides against an invading force, the city enjoys a high level of security against such threats, which explains the inhabitants fierce opposition to anything that they perceive might jeopardize it.