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It is also known as *Hanshuishi* (Calcitum, 寒水石). Its texture is transparent and clear. Some people grind and carve it into pillows for use during the summer months. For medicinal use, it must be calcined. However, merchants sometimes calcine it and mix it with *Nifen* (Calomel, 腻粉) to pass it off as genuine; this requires careful examination. Tao Hongjing stated that the best quality is that which can form ice in summer. If this were true, no one in the world could obtain it, suggesting his statement was inaccurate.

The two texts debate without resolution, and the underlying principle remains unclear. A detailed examination of the *Shennong Bencao Jing* (Classic of Materia Medica) reveals no original mention of *Fangjieshi* (Gypsum Fibrosum var.). This confusion likely arose because the *Tang Bencao* (Tang Dynasty Pharmacopoeia) notes that *Shigao* (Gypsum Fibrosum) and *Fangjieshi* are largely similar. Consequently, later generations became confused. The classic states: "Grows in the valleys of Qi Mountain, as well as Qilushan and Lumen Mountain. Harvested at any time." From this, we know that those from other regions are not authentic. The current *Tujing* (Illustrated Records) includes specimens from Fenzhou, a point overlooked by previous scholars. The classic states that those with fine grain and bright white luster are superior. Thus, if they lack these characteristics, they are not *Shigao*. In the entry for *Lishi* (Magnesite), the classic mentions: "Similar to *Shigao*, following the grain and being fine," which clarifies this further. Among what is currently called *Shigao* and *Fangjieshi*, how many possess the qualities of following the grain, having fine texture, and exhibiting bright white luster? Those with these qualities are *Shigao*; those without are not. Furthermore, only those from the specific regions mentioned in the classic should be used medicinally. Others represent biased views and should be disregarded. In Zhang Zhongjing’s *Baihu Tang* (White Tiger Decoction), its efficacy is miraculous. The corrected annotations after Zhang Zhongjing’s *Shanghan Lun* (Treatise on Cold Damage Disorders) state that in late April, when the weather is hot, *Baihu Tang* is indicated. However, climates vary across regions, and annual qi movements differ. Given these geographical and temporal variations, although the theory is elegant, one must exercise double caution. For cold damage febrile diseases, or after profuse sweating, if the pulse is flooding (*Hong*), the mouth and tongue are dry, there is headache, and extreme thirst persists; or if suffering from summerheat, with body pain and fatigue, *Baihu Tang* is invariably effective.

Its color is light purple, the stone surface is cracked and rough, and it can attract needles and iron. It is commonly called *Tieshi* (Magnetite, 铁石). It nourishes kidney qi and replenishes essence and marrow. It is used for deafness and blurred vision caused by kidney deficiency. For medicinal use, it must be heated red-hot and quenched in vinegar. Its black variety is called *Xuanshi*, which is simply the black form of *Cishi* (Magnetitum, 磁石). It is mostly smooth and clean. Their therapeutic effects are largely identical with minor differences, and they should not be divided into two separate categories. When the tip of a needle is ground, it can point south, but it often points slightly east rather than due south. The method is to take a single thread from new cotton wool, attach it to the middle of the needle with a piece of wax the size of half a mustard seed, and hang it in a windless place; the needle will then consistently point south. Alternatively, threading the needle horizontally through a rush pith and floating it on water also makes it point south, though it often deviates toward the Bing position. This is because Bing belongs to Great Fire, and Geng/Xin Metal is controlled by it; thus, this phenomenon results from the mutual感应 (interaction/influence) of physical principles.