So as you probably all know, valuing gold is easy. You just need an accurate postal scale to weigh it, and most online calculators for scrap gold will tell you what each gram of each particular carat weight costs. To get the most out of your return, don't sell gold to a jeweler, try to mail it to a metal refinery directly, or to anyone else who gives you at least 95% of the scrap value. I have a wholesale account with a scrap dealer so I can sell it for you if you like, I get 98.5% of that days's spot value, and I trade in old gold as part of payment for items in my shop.
Here is an online 'Scrap Gold Calculator' by grams
Diamonds are also not that hard to evaluate - in principle at least. You do need to know the color (G, H,...) the carat weight and the clarity (VS, VVS, SI...). Then you can look up comparable diamonds on eBay. Remember too that diamonds can get chipped over time, and those can devalue the gem.
Diamond melees fetch very little in the market. They are bought at 1/2 or even 1/4 of list price (they're called "breakout diamonds" and the price is "breakout price"). Diamond setters buy breakouts fairly frequently.
Most difficult to evaluate are colored stones, especially the most well known three: ruby, emerald, sapphire, and even more especially, when these were bought at department stores or jewelry chains (excepting high end retailers such as Tiffany's, Cartier, etc.). Most of the colored gems in commercial jewelry are treated, and not just heated but also diffused, glass filled, resin filled, etc. Only a lab can tell you if that's the case, and that's why I, for instance, will not buy any such items except for the gold value - and most jewelers do the same.
On the other end of the spectrum, citrine, pyrope garnet, amethyst, peridot and topaz have little to no value per carat and are usually scrapped because once they are worn a few times, they'll scratch and chip. In the market, their values is treated as basically zero, even if they are "master cut".
However, since many of my customers are savvy gemstone shoppers, you should know that if you bought a collector's gem from a reputable source and it is in good shape and good size, you don't need to undersell yourselves. Don't take that stuff to a jewelry store, most retailers are not familiar with collector's gems because that is not their business. So you will buy below value. Go to eBay, Loupe Troop, or any other site that lets you sell your stuff, describe it accurately, add disclaimers if need be (i.e. that you are not a jeweler or that it is pre owned), offer a return period (make it short but DO offer it), and you have the best chance at recovering your value - or at least more value than you would have gotten elsewhere. Take your time though. If you are desperate for cash you have to sell lower. Case in point: many gem dealers answer the a customer's question "what is this worth if I sell it" with "are you in a rush to sell?". And a "yes" or "need money now" answer yields a low ball offer.
Finally, are there some gems that I think you might consider holding on to, at least at present. Right now, I would recommend that if you're not desperate, you hang on to any Mahenge spinel, even small, if it is that neon color and even reasonably clean. Half the mine is shut down now for lack of production, and the other half is producing very little, and only 5% or so neon color. Tsavorite production is also down. Sapphire is worth keeping if it is heated only or not treated, eye clean, at least 1/2 carat, and a nice saturated medium to a more saturated royal blue, or a color changer.
|Neon Mahenge Spinel available on my Etsy Shop.|
What about other gems? Well the spessartite mine is only not producing right now because of local unrest, not because of lack of material (the story is more complicated, but I'm skipping it here). Other gems, i.e. chrysoberyl, are not known enough right now, but they are not declining in value. Neither is tourmaline, and chrome color or bright blue or bright pink are always desirable. Obviously Paraibas are worth keeping. Other tourmaline colors have much less value and production is constant. Right now there is a shortage of aqua, but I don't know why and it might be temporary. Heat treatment doesn't affect aqua prices. Emerald I don't know enough about. It is my understanding that emerald rough is being mined steadily, but the mines are controlled by investors and/or government. Good material is available only with connections. I see very little that's both good and untreated on the market, so if you have a gem that is both, and you haven't scratched it, then you might consider holding on to it as well.
Personally, I keep very few gems. In the end I always sell because that's what I do. Sometimes I keep smaller pieces of rare gems because I like having something left that I like. But I have kept some Mahenges, and I am keeping some sapphires, some mint garnet and some tsavorite. But spinel (Burmese red, Mahenge pink) and sapphire (blue) are my strongest bet.