A boart is a diamond of very poor quality that is used in powder form as abrasive.
Here we are talking about very small diamonds or even diamond dust which can be used by diamond polishers to polish other diamonds. When you think about it, it is really the refuse of the diamond after it has been polished that can still be used to polish a high quality diamond in its desired shape. Nowadays diamond dust is also used for many different kinds of spa treatments. For example, to reduce wrinkles.