Ectoplasm

Most apparitions are formed of a substance called ectoplasm (Romantically sometimes called "The Soul"), no matter the special properties of the ghost, ectoplasm has specific rules that cannot be changed. In some cases ghosts have left marks known as "ectoplasm burns" on physical objects, these seem to be places where ghosts have emerged or appeared, however this does not always leave an ectoplasm burn.
 * 1) Ectoplasm cannot come close to substances such as salt, iron, or silver. If it is forced into contact with any of these substances, it will undergo a reaction similar to burning or boiling.
 * 2) Contact between living flesh and a substantial concentration of ectoplasm will cause a condition in the living creature called "Ghost Touch". This is often fatal.

Inactive ectoplasm (ectoplasm that has been separated by the main ghost by some damage to its form by a ghost repelling substance) cannot cause Ghost Touch, however it does appear to cause a burning reaction when in contact with human skin when in this state, so contact with it is not advised.

Inactive ectoplasm has also been known to cause lasting stains on clothes or other fabrics, as well as Rapier blades. This has been compared to egg stains visually.

Some people believe that ectoplasm exists in a harmless, invisible state inside the living as well, acting as their souls.

Ectoplasm can have some strange effects when harnessed by people for their own means, as evidenced by Marissa Fittes experiments to stay young.