When you buy from reputable sources you generally don't need to be worried about it unless it got past them. Any type of line or dot can be toxic or not. Sometimes toxicities can show up on a stone that has been fine in the past. Try to get a guarantee when you buy a stone on toxic inclusions. Toxicities can range from a stone that is acidic to sand particles. An acidic stone will most of the time have a slightly sour sulfuric smell to it and can turn a blade black in a matter of minutes and a sand particle will grab a chunk of steel out of your fresh bevel while your making laps.