The katana, fully forged, now goes to a skilled sword polisher, who may spend more than two weeks honing the sword's razor-sharp edge. He meticulously rubs the blade with a series of grinding and polishing stones, some valued at more than $1,000 each and often passed down through families for generations. Sometimes called "water stones," these tools are typically composed of hard silicate particles suspended in clay. As the clay slowly wears away during use, more silicate particles are revealed, guaranteeing excellent polishing quality throughout the life of the stone. Each consecutive set of polishing stones contains finer and finer silicate particles and removes less and less of the steel.