Because carbon isotopes have a half-life, carbon-based artifacts can be roughly dated. One recent example was the carbon-dating of the Shroud of Turin to help establish its authenticity. Paleontologists use carbon-dating on bones found at burial sites to determine the age of the settlement. In botany the process can often aid in determining plant species, how old tree trunks are, and similar questions. The process is painstaking ad fraught with error; for example, the Turin investigation may have been polluted by pollen from medieval times that fell on the fabric. It also is only applicable on carbon forms; for example, fossilized trees cannot be dated (unless only partially) because in fossilization, the vegetable, carbon-based form been displaced by silicon-based material.