■ The runes were symbols of power and knowledge. Emphasis on the word symbol. The medium (such as wood or bone) was important, sure, but the power came from the engraved symbol. A wood piece without a symbol can hardly be called a rune. To be a rune mandates that a symbol be etched, engraved or whatever on the media.
■ Just so we are clear, if the ancient Norse people wanted a blank symbol, they would have engraved it. Look at this sentence and how their are spaces between the words. We use the lack of a symbol (by hitting the space bar) to impose a blank symbol between words. This may seem obvious but let us not impose our practices on people who lived a millenia ago. Why you ask? Because the Germanic people did not use spaces to separate words. They had a symbol to represent a space. (It was what we would call the colon). Here is a great example from an ancient stone carving in Sweden:
Photograph from Simone at http://hem.bredband.net/sim1/swe/mal/si ... rd_01.html (a great site on the Sigurd Carving in Sweden, Sigurd Fafnirsbane is one of the greatest heroes from the Viking era, the Norse equivalent of King Arthur).
So, it the Norse wanted a blank image, they had a symbol for it. The notion of the blank rune, I suspect, would be an alien concept to them.
