Sometimes a cavity can result from normal everyday wear. If a diamond has an inclusion extending from the surface into the stone, a sharp blow can dislodge it, leaving a cavity. If the inclusion lies just below the surface and next to a fragile area such as an extremely thin girdle, it is also very possible for the area to break away, again leaving a cavity.
Many times a cavity will fill in with oil and dirt, thus acquiring a darkish color, which can easily be removed with a strong acid boil (as discussed later). A dealer may look at the cavity, thinking it is a much worse inclusion than does a more knowledgeable buyer who buys the stone and boils it, revealing only a small cavity that is difficult to detect under 10x. On the other hand, there can easily be dark crystals hidden behind the cavity, which all the boiling in the world won’t cure. The idea is to look closely at all possible angles with your loupe, trying to determine the depth and surface size of a cavity before commiting to an offer the diamond. In doing this, you will often discover that the cavity is easily removed, whereas the owner of the stone either doesn’t want to deal with it or is unaware of its potential.