This next topic applies whether or not the gemstone in your jewelry is a diamond. Still, since the series has been Understanding Your Diamond Appraisal, we'll focus on Gold and Platinum today. We'll come back to Silver, Palladium, Cobalt, Titanium, Tungsten, Stainless Steel, and Tantalum at another time.
First, gold is available in a few colors, most notably yellow, white, and rose (green, blue, and purple exist, but you won't see them much). Pure gold, or 24 karat gold, is a bright orangy yellow and is incredibly malleable (able to be beaten into sheets) and ductile (able to be drawn out into thin wire). Thus, gold is not often used for jewelry in its pure 24K form, at least not in the US.
The most common karatages you will see are 18K (75% pure), 14K (58.5% pure), and 10K (41.6% pure). The remaining parts of the alloy, typically copper, zinc, nickel, silver, palladium, and others, impart the final color of the jewelry. If you find yourself allergic to white gold, it's not the gold but the nickel in the alloy. If yellow and rose gold turn your finger black, then you have a sensitivity to copper. Gold itself is inert and nonreactive.
White gold is typically plated with rhodium, a metal related to platinum, to give it a bright, white appearance. Depending on body chemistry, chemical exposure, and wear, this plating will need to be reapplied every year or two to maintain that finish. If you purchased a ring that was bright white, but it's been a few years, and it appears to be "turning yellowish," what you're seeing is the plating wearing off. It's an easy fix and something that I might advise before you bring your pieces for appraisal.
Platinum is a dense metal that is white (well, metallic gray) all the way through and doesn't require any maintenance to stay that way. While platinum is dense, it is easily scratched and acquires a patina after a year or so of wear. The patina is the soft finish you might have seen on Art Deco or Edwardian pieces that were very often made in platinum.
Platinum is also different from gold in that it has no memory. I know that sounds weird, but if you take a gold wire and tie it in a knot, it will work itself free, but platinum will stay just where you put it. This is both great and a problem for setting, since when a jeweler sets a diamond in the center of an engagement ring, the prongs will go right where they are placed-- unless the ring takes a blow and one of those prongs is moved or lifted. So, while platinum might be the high-end metal of choice for diamond settings, it certainly isn't foolproof.
Finally, platinum doesn't wear away over time in the way that gold and other metals do. Don't get me wrong, if it is sanded or polished or rubbed against other jewelry pieces, it will wear down, but in general, it wears much, much slower. That's one reason it shows that patina-- when it scratches, the metal is moving around instead of being taken off. Again, a positive for longevity, but if you're someone who loves high shine, it could be a negative.
As always, I hope this has helped you understand your appraisals, and if you have questions, please don't hesitate to ask!