The esoteric substance called muppu is particular to Siddha medicine and may be considered as Siddha’s equivalent of the "philosopher’s stone." Its preparation is hidden in secrecy, known only by the guru and taught only when the student is deemed qualified to accept it. It is generally thought to consist of three salts
(mu-uppu) called puniru, kallupu, and vediyuppu, which correspond respectively to the sun, moon, and fire. Puniru is said to be a certain kind of limestone composed of globules that are found underneath a type of clay called Fuller’s Earth. It is collected only on the full-moon night in April, when it is said to bubble out from the limestone, and is then purified with the use of a special herb. Kallupu is hard salt or stone salt, i.e., rock salt, which is dug up from mines under the earth; or is obtained from saline deposits under the sea, or it can be gathered from the froth of sea water, which carries the undersea saline. It is considered to be useful in the consolidation of mercury and other metals. Finally, vediyuppu is potassium nitrate, which is cleaned seven times and purified with alum.