Diagnostic artifacts are often collected by archaeologists when a site is recorded. Diagnostic artifacts are those with known time periods of production and use. The range of dates associated with particular artifacts is based on data collected from many archaeological sites, including radiocarbon and tree-ring dates and the composition of an artifact assemblage. The value of this type of dating depends on the length of time a particular style of artifact was used. Some types were used for only a short time while others may have been used for many hundreds of years. Artifacts that were used for short time periods are those most useful to scientists.
Perhaps the first and most important question about any site is when were people here? Artifact cross dating is the only way to answer this important question on most sites. This is because radiocarbon dating requires organic materials that do not preserve well on sites that are exposed to the elements. This is just one reason that it is important that visitors to archaeological sites leave artifacts where they find them. When someone takes an arrow point or piece of pottery from a site they take something far more valuable than the object, they rob the site of the most important information of all - when were people here?