Sekkiseki (殺気石, Spirit Reducing Stone) is a special mineral with properties that allow it to repel all Reiryoku.
Sekkiseki is present in large quantities within Soul Society, particularly in the wall that surrounds Seireitei. The mineral projects a unique wave of spiritual energy from its cut surfaces, enveloping the city with a spherical barrier called the Shakonmaku (遮魂膜, Soul-Warding Membrane). Because of the stone’s properties, any Reishi that passes through the barrier is instantly vaporized. However, it is possible for Reishi of sufficient density to pass through, albeit with difficulty. The Shakonmaku is even capable of preventing direct transportation into and out of Seireitei.
The walls of the Shishinrō (四深牢, Four Deep Prison) within the Senzaikyū are also made of sekkiseki. Its presence interferes with the prisoners’ ability to sense Reiatsu. In addition, the constant exposure to the mineral drains their Reiryoku, reducing it to its absolute lowest level. This leaves them particularly vulnerable to powerful Reiatsu.