My finest car boot moments were not quite so lucrative but mostly involved buying Atari, Commodore and vintage Nintendo consoles which at one time were going for a few pounds as people emptied their attics.
The £657,000 diamond was 26 carat - the word whopping doesn't quite go far enough when it is also used to describe a burger. Our guides go as far as 2 carat diamond rings, but a 'normal sized' engagement ring is certainly less than 1 carat (mine was 0.35 carat or thereabout).
As you've probably gathered, one 26 carat ring is worth a whole lot more than 26 x 1 carat rings. The cheapest new 1 carat ring on our guide is about £1000 (I won't patronise you by doing the maths for you). A decent one is usually much more expensive.
Rather obviously, larger diamonds are rarer than smaller ones and you can't exactly stick two diamonds together or melt them down, as you could with two chunks of gold.
Which brings me to my main point - be very wary of the letters TDW if you see it on a diamond engagement ring which appears to be a bargain. It stands for total diamond weight, which means that they have added together the weight of all the tiny diamonds on a ring to come up with one impressive total. Usually such rings are called cluster diamond rings, but it isn't always made clear.
A cluster diamond ring with a TDW of 1 carat is worth significantly less than an engagement ring with a single 1 carat diamond.
Don't get me wrong, there's nothing wrong with cluster diamond engagement rings. In many cases they look rather nice and give a fantastic sparkle. They're also a good budget option which can look worth much more than its price tag.
You'll find big name jewellers will label very clearly but be vary if there's a very limited description or if it's secondhand and hasn't been well photographed.
For advice on buying second hand, see our guide.