Oil of Saints is described as:
- an oily substance which is said to have flowed (or, in some cases,
still flows) from the relics of burial places of certain saints;
- the water that flows from the wells near their burial places;
- or oil and/or water which have somehow come into contact with
their relics.
In the first two cases, the "oil" is probably groundwater
containing minerals that give it a smooth, oily feeling
against your fingers. As for the last case, in the 4th century it was
customary to pour oil over the relics or reliquaries of
martyrs, and to gather the oil in vases or soak pieces of cloth with
it. This oil was called oleum martyris, and was given to the
faithful as a remedy against sickness. The Catholic Encyclopedia is very careful to point out that the
cure was not believed to be wrought by the oil itself, but through the
intercession of the saint with whom the oil had been in
contact. Eventually, the custom extended to the relics of saints who
did not die as martyrs.
The most famous of the Elaephori, or oil-yielding saints, is probably Saint Walburga, who rests under a stone slab in a church in
Bavaria. The oil, which has been analyzed and shown to be nothing
more than water, is caught in a silver cup and distributed in small
vials by the Sisters of Saint Benedict who run the church.
Other well-known Elaephori (this is by no means an exhaustive list):
www.newadvent.org
Webster 1913