Menthol is steam distilled from Mentha Arvensis, a plant in the mint family. It is a clear, colorless needle-like crystal with a typical peppermint odor. It is all but insoluble in water and glycerin, but it is completely soluble in alcohol and essential oils. Due to its low melting point (107°F), it is easily solubilzed in slightly warmed fixed oils. This makes adding it to ointments, balms and salves very easy. It has an analgesic and cooling effect in these formulations.
Menthol Crystals are often used in sports rubs ... such as Geri's Menthol Sports Salve. (Recipe can be found here.) It is said by some to relieve stiffness or sore muscles when used as a salve.
Caution: Please do your own research on the Essential Oils you choose to add and/or consult with your health care provider before use!