Thanks for the comments chaps.
The knot was mahogany veneer and cooperative lemonade can.
I stopped about 2mm oversize and started going through the grits from 240 - 400, and the wood was gradually getting darker and darker as it picked up aluminium.
I then took it to size with the skew and it was ok, a quick rub with wax and I was done.
It was a gift for an elderly lady whose husband had died some 2 years ago. She asked if I wanted the wood that was lying in the shed as she was moving/downsizing.
There was one or two pieces of wood that will be useful but the best find was a box that had contained a marquetry project and there was still some veneer inside.
I made the pen from wood and veneer but had to add the aluminium to fill the kerf.
She was over the moon with the gift as were her daughters.
take care
Don W