The Arcane Focus determines the range and width of the Arcane Bores mining area. There are several different types of Foci, but the others are only gained through research. these include the same types as runic essence; air, water, fire, earth, etc. While the Arcane Bore is in operation the focus will slowly suffer damage and will eventually break. i'm not sure right now, but maybe the different types apply certain traits to the arcane bore's procedure.
The Fire type makes it work slower, but is more efficent for Vis and Taint.
The different types of arcane focus can be infused. The infusion recipe is simply a vis crystal whose type you wish to convert the arcane focus into e.g. arcane focus and firey crystal for an "arcane focus: fire". However, as said in the previous paragraph, they reqire research for most types.