Oh dear, you’ve just experienced the jeweller’s nightmare. Yes,
diamonds can “burn”, and that’s what has happened. There are a
number of reasons that this might happen.
From a working jeweller’s experience, the most common is when the
diamond’s surface has not been protected with a coating of flux.
The next most common cause is from a cooler draft of air suddenly
changing the temperature of the diamond’s surface. The zephyr-like
breeze of someone walking behind you as you are retipping the
diamond ring is enough to "burn’ a diamond if it is not protected.
The diamond can only be restored by a full recut and polish.
To prevent such a disaster befalling you again, coat the diamond
with a layer of protective flux. In Australia, we use a thin
mixture of boracic powder and methylated spirits. Mixed to a thin
consistency, this is painted over the diamond then set alight to
burn out the methylated spirits. This leaves you with a thin coat
of boracic evenly over the diamond. when you begin to retip the
claws, the boracic melts into a glassy protective coating. When
you’ve finished, it can be dissolved off in hot water and pickled
to remove any remaining oxidisation.
I’ve also used a proprietary brand of gold-soldering flux called
Auflux quite successfully for the same purpose.
From a gemmologist’s point of view, diamond, when heated will
eventually convert to graphite. Eric Bruton’s “bible” on diamonds
refers to this happening at around 800 to 1700 degrees C. He goes
into a lot of detail about this in Chapter 19, pp 421-442,
describing the processes that cause the surface to “burn” to a
frosty-white appearance. (Bruton, Eric FGA (1979) "Diamonds"
N.A.G. Press, London.)