The fireplace can be crafted from natural materials (e.g. Stone/Granite/Marble) which have formed over many millions of years, in many cases over 300 million years. Extracted directly out of the earth containing fossils and layers of organic material which, when worked, form the exquisite beauty of stone that ancient nature has created. Therefore you can expect to find markings and veining running throughout  the material  which would either not be present or removed in a man-made manufactured product, which is then seen as flawless and consistant.  Both have their place, those of us who appreciate natural materials in their homes and undeniably those who wish for products made from manufactured materials without imperfections (e.g. Corian).