Very briefly, when tanzanite is mined, it is almost always grayish or brownish. Once in awhile a "rough" will come out purple or blue, but for those that don't, heat treatment is there. This treatment brings out the beautiful blues and purples tanzanite is becoming so famous for. You need not worry about this. The treatment is permanent, therefore totally acceptable. Heated stones sell for the same price as untreated ones. We just thought you'd like to know that the practice exists so when a jeweler tells you about it, you won't be caught off guard. You can also use this as a good test for any unknown jeweler. Ask him if his tanzanite has been heat-treated. If he says no, you might want to consider another dealer. If he says yes, you not only have an honest jeweler, you probably will have won new respect from him and he will treat you accordingly.