Donna, Diamonds in nature, at least those in the yellow to white
range of color, almost always have a bit of nitrogen trapped in the
crystal structure. That's what gives them their yellow color. Note
that this isn't something that changes under normal conditions.
White or near white stones simply don't become more yellow with
time. Doesn't happen.
If your diamond now appears more yellow than it used to, there are
several possibilities. The most likely is that there is some dirt or
something of the sort coating the stone, perhaps only in a small
area, which is causing the appearance of a shift in color. A good
cleaning, perhaps using professional equipment such as an ultrasonic
cleaner (not the cheap consumer level "sonic" cleaners. Ultrasonic
cleaners are quite different, and much more costly. AND much more
effective) and steam cleaners might be needed to get off some
stubborn deposits, especially if they're trapped between the stone
and it's setting.
Another possibility is that the stone is the same color it's been,
but wear and tear on the mounting, changing the appearance of the
mounting, may affect how the color of the stone now appears. For
example, if the mounting has yellow gold on it, which was highly
polished, but now is not, the more visible yellow color of the gold
may fool the eye in thinking the diamond is also more yellow. Some
gold mountings, even yellow gold, may be initially plated in parts,
with rhodium, a very white metal. If that wears off, what then
remains to reflect light into or around the stone, may be yellower,
again changing your perception of the stone's color.
Another, but quite unlikely possibility, is that the stone is it's
same color now, but that now you see the color, whereas when it was
new, it may have been masked. Some few unscrupulous sellers have, now
and then, altered the appearance of faintly yellow diamonds by
painting on, usually as a thin line just under the girdle of the
stone, on the back, a transparent but very lightly blue dye.
Normally it's not easy to see, but it does reflect bluer light into
the stone, causing the stones apparent color to be whiter. This
coating isn't permanent, usually, especially under the attack of the
aformentioned professional level cleaning equipment, so a good
cleaning by some other jeweler might remove that coating, leaving
your stone looking as it really is. Please note that this practice
is very uncommon today, so this is kind of far fetched as a
possibility. Still, it's there, and every so often one hears of
someone trying to do this still.
Light sources in your home can also make a difference. Some lights,
especially fluorescents, dont' show yellows very well, while others,
incandescents, increase the yellowish appearance of things. If
you've changed the way you light your environment, the apparent color
of your diamond may have followed suit.
With all that said, again the most likely explanation is just that
your diamond needs a good professional level cleaning.
Hope that helps.