Hi, Answerthis !
There's a very good discussion of this at:
".... During the manufacturing process, the culet is often polished as
a flat facet so that it does not get chipped as the other facets are
polished. These days the cutter usually ?closes' this facet to a
point, but sometimes it remains as a small extra facet; this is not so
bad because it protects the culet from chipping as the diamond is
handled and while the jeweller is setting the stone. In fact the AGS
originally down graded a diamond if the culet was closed, but changed
this policy because of demand from Asia for diamonds with closed
culet'. In some cultures an open culet is said to allow evil spirits
to get into the diamond (no kidding!).
A diamond behaves as a window if opposing facets are parallel, and
this is exactly what happens if the culet is too big, because the
culet is parallel to the table. The result is that you can see right
out the ?hole' the culet appears to make in the center bottom of the
diamond. If you have ever seen a large old cut diamond you probably
know what we mean.
The culet size is listed on a cert and your diamond's culet should be
pointed (no culet), very small, small or medium because these are not
visible to the naked eye.
Large to extremely large culets may be visible to the naked eye and
can look like an inclusion."
I take it you are mostly concerned with the appearance of your diamond
when you are wearing it. If you put the ring on your finger and look
straight down at the table of the diamond (the flat front face) you
will be looking through the culet. Most likely you won't see anything
untoward, because there isn't sufficient light between the back of the
diamond and your finger to create a problem - the light that reaches
the table is more likely to be coming in through the sides of the
stone if the setting allows this.
What you can't see, no-one else will - and besides, just how often is
someone else going to be staring straight down at your diamond at
precisely that angle ?
If you don't like what you see, then either choose another diamond or
choose another setting.
Try the same thing when it is not being worn for a feel as to whether
the size of the flat culet is really significant - but remember, who
is ever going to examine it that way, other than perhaps a jeweller ?
diamond "cushion cut"