The main thing with oil stones is to maintain their flatness and this is best done on a regular basis through different means. Leaving them only makes the process less appealing and often results in stones not living out a full life.
Man-made oil stones are generally best preserved with a steel diamond plate but as you can appreciate although the stones are cheaper to buy it can be more expensive to lap them flat again using other abrasives. These are also less refined than natural stones which hone to a finer degree ensuring that your tools have the best possible edge.
There are many types of natural stones many of which are used for honing cut throat razors but those such as welsh slate or Charnley Forest are often found in old tool kits and tend to lap flat again fairly easily with the use of abrasive cloth or paper on a flat surface and therefore cost less to maintain.
Both types play a roll in a woodworkers tool kit but generally when an edge has been established the use of the natural stone is important to refine the edge so the tool stays sharper for longer. Slate varies in grades but the finest do offer real value and probably why so many old timers had them.
Why buy an old oilstone? Well there is two parts to the reason, firstly good value for the stone which can mostly be lapped flat if needed but especially if they comes with a box as these can take at least couple of hours to make, buying a new box (which are rubbish by the way) for a tenner rarely houses an oilstone properly without tweaks by which time you might as well made one.
So next time you see an old stone in a box that’s slightly uneven don’t discard it just lap it with the methods above.
It may be worth mentioning to new woodworkers that old boxes often come with a couple of pins in the bottom to stop them moving on the bench when in use.