Sentence:  The <WSD>culet</WSD> looks like a point at the bottom of the stone, but it is normally another facet—a tiny, flat, polished surface. This facet should be "small or very small". A small or very small <WSD>culet</WSD> won't be noticeable from the top. Some diamonds today are actually pointed. This means that there really is no <WSD>culet</WSD>, that the stone has been cut straight down to a point instead. The larger the <WSD>culet</WSD>, the more visible it will be from the top. The more visible, the lower the cost of the stone. Stones described as having a large or "open" <WSD>culet</WSD>, such as old-European or old-mine cut diamonds (see chapter 5), are less desirable because the appearance of the <WSD>culet</WSD> causes a reduction in sparkle or brilliance at the very center of the stone. For the same reasons, a broken or chipped <WSD>culet</WSD> will seriously detract from the stone's beauty and significantly reduce the cost. 
Meaning of the word marked with <WSD> tags:  A small, flat face often cut at the base of a brilliant-cut gemstone.