Hello,

So I've gotten to the point where I've learned about the 4 Cs-- the
four characteristics used to judge the value of a diamond.

But what I want to know is the common formulas used to calculate a
diamond's actual worth.  At this point I don't even know if it is
linear in each category, or whether each category is equally
significant in determining the final monetary worth.  I would love to
have the formula to make it easier to shop smarter.  Thanks.

After some research, I don't think there is a formula at all.  Diamond
prices are simply market driven.
The Rapaport of New York is the most respected diamond price list and
can be found on www.diamonds.net.  You must sign up to their service
to see the list (including all variables of the Cs) but it looks like
signing up is free (they probably just want to send you a lot of
diamond advertisements in the future via email).

There likely isn't any straightforward formula to it.  Judging the
"worth" of a diamond is a tricky issue anyway.  All diamonds are
hideously overpriced and unless you are planning on selling it a
couple years down the road, its appraised value really isn't that
important.  Shop around some and then pick a diamond that looks nice
to you, that you think your lucky lady will like, and that you can
afford.

Find a jeweler you trust.  Only buy a diamond that is certified.  Ask
to see the papers.  A good jeweler will let you get an independent
apraisal.

I recommend buying the main diamond separately, then buying the
setting and having the stone mounted.

Jewelers live and die by their reputation.  Find one with a good
reputation a you should have a good experience.