Final answer:
Tool marks are considered individual evidence when they have unique characteristics such as distinctive chips or breaks that can link them back to a specific tool and by extension to a crime scene.
Step-by-step explanation:
Under what circumstances can tool marks be considered individual evidence? Tool marks can be categorized as individual evidence when the marks contain unique characteristics that can be traced back to a specific tool. Most commonly, this occurs when the tool has chips or breaks in it, as these defects can leave distinct patterns that are unique, much like a fingerprint. For instance, if a tool used in a burglary has a chipped edge and that same irregular pattern is found on a window frame, it could serve as individual evidence linking the tool to the crime scene.
While new tools or those with little wear might seem less likely to leave individual evidence due to lack of unique characteristics, they are not necessarily disqualified. Over time with use, they might develop distinctive wear patterns or acquire specific damage that can also contribute to creating unique tool marks. It's the unique characteristics, regardless of whether the tool is new, used, with little wear or has obvious defects that define individual evidence.