First, I have to mention that I have noticed that the size of a
prong setting that a supplier lists for a given size stone may vary
from each supplier. Some are larger, some smaller. That said, let’s
assume that the head we’re talking about came from Stuller, a major
US supplier. A 6.0 mm round stone will sit right at the top of a 6mm
head. After you cut the seat, the stone will be set near the tips of
the prongs. If you want to set the stone high like this, that’s fine.
If you wish to set it lower, I would use a ball punch to slightly
spread the prongs apart. The punch must fit low in the setting, near
the base. I want to spread the setting at the base of the prongs, not
just bend them out.
If I had a stone like you have, 6.5 mm, I would choose a 6.0 mm head
and spread the prongs. If I wanted to set the stone low, I might
choose the 7mm head instead. A 6.5 mm stone is a calibrated, and
common, stone size so you should not have a problem finding a head to
fit. Let’s say you have a 6.7 mm stone: I would still choose a
smaller head and spread it.
I once worked with a setter from Van Cleef and Arpel in New York. He
routinely spread every head that they used. He preferred them with
slightly more seat under the stone and a slightly thicker prong at
the girdle. I started setting my prongs that way, and I must say that
I have had fewer loose stones because of this. It is a VERY SLIGHT
difference, but it seems to give a lot more strength to the prongs.
Here’s another reason to vary the angle of the prongs: Let’s say
that you have to construct a 5-stone ring. The center stone is 8.0
mm, the side stones are 4.0 mm and 2.0 mm. Draw a circle that is the
size of the ring you wish to construct. Draw the outside diameter,
then draw the stones in place. Now, draw the angles from the edge of
each stone to converge at the center of the ring. You will notice
that not all the angles are the same. If the ring is a size 4, for
example, this will be really exaggerated. In my work, I will adjust
the angles of each head so that they all converge at the center. I am
doing high-end custom work, and I charge accordingly. If I worked at
a volume discount store, I would merely file the edges of each
setting so that they fit in a roughly graceful arc.
I realize that I may not have answered your original question, and
have opened up more questions along the way so that we are more
confused now than we were before, but at least we are confused on a
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