The Mishna (15b) mentioned the tradition that had been handed down that the aron had been hidden away towards the end of the first Temple period. Our Gemara teaches that according to that tradition, several other items that were on display in the Temple were concealed together with the Ark. They included the container of manna, Aharon the high Priest’s staff and the flask of the shemen ha-mish’ha, the oil used for anointing.
The shemen ha-mish’ha was made from afarsimon (which may be identical to the tzari mentioned in the Torah), which was, apparently, the plant Commiphora apobalsamum. This is a small tree or shrub that stands from 10 to 12 feet high, with wand-like, spreading branches. The best perfume that can be extracted from it drips from the seeds, but it is usually produced by boiling the branches. The oil that is extracted from this plant was occasionally used as a medicine, but more as incense or perfumed oil. The afarsimon was considered so valuable that at one point it was literally worth its weight in gold.
The shemen ha-mish’ha was used to anoint kings and high priests. The Rosh points out that the need to anoint the high priest is from a clear passage in the Torah (see 30:30), but there appears to be a prohibition to use the oil on any other person (see 30:32). How was the decision made to use this oil on kings, as well?
He answers that the Gemara in Megilla understands that it is only forbidden to use this oil on a normal person. The king is not simply an adam (man) and therefore he does not fall into the category of the prohibition.
According to the Gemara in Horayot (12a), kings were anointed by putting the oil around their head like a crown. The kohanim had the oil put on them ke-min key, or, as the Gemara explains, ke-min kaf yevani – like a Greek chi – what we would call the shape of the letter “X”. Since there is no Hebrew letter that is similar in shape to an “X,” many suggestions were made by the commentaries over the years about its appearance, given that Greek was no longer commonly used and people did not know what the letter looked like.
According to the shape of the letter as we know it, it appears that the oil would be placed on the forehead of the priest, beginning between his eyebrows and spread diagonally towards his head, making the shape of an “X”.