Bold Artifacts are special items that your hero may come across while questing. Your hero will not find these types of artifacts very often as they are quite rare, however, these items will fetch a higher price than normal artifacts while conducting trade with a merchant.
The Bold Artifacts category lists bold artifacts with an article on the GodWiki.
The List of Artifacts also denotes those which are bold artifacts, and includes many for which an article has not yet been written.
How to recognize
The names of these artifacts appear Bold.
How to use
The god can urge the hero to use any bold artifact in the same manner as any other artifact; although there is a chance the hero will lose (destroy, etc) the artifact in question, so it doesn't make any sense to lose a bold (expensive) one when there are cheaper alternatives.
It is worth noting that a few Bold Artifacts have special uses.