I have seen a rash of old European cut diamonds come across my desk lately. They are truly beautiful diamonds, but they don't have the value of the modern brilliant cut. Old European cut diamonds need to be re-cut and  polished by a diamond.  They will typically lose 20 to 30% of their weight once the cutting process is completed. Many times this reduces the size of the diamond from, for instance, just over 1.00 carat to just under 1.00 carat.  We try to re-cut and polish only the diamonds that don't lose an extreme amount of value unlike the example above.  Typically if you have a 1.75 carat European cut diamond and you need to have it cut to an a modern brilliant cut, we should be able to keep the diamond weight over 1.00 carat after recutting, thus keeping the value high.  The value you will receive is based on a modern brilliant cut, after re-cutting. Therefore you can't really compare the price you'll get for a 1.75 carat European cut with a 1.75 carat brilliant cut. Your value will be based on the post re-cutting weight.  You don't have to worry about the re-cutting process.  The buyer will worry about that, all you need to know is that the value of your diamond is based on its desirability as a modern brilliant cut.