Color, cut, clarity—these are just a few of the qualities that determine a stone's worth. Here's what to look for when investing in the classics (and a few lesser-known varieties, gleaming with next-big-thing potential).
Hue is the most important attribute in a ruby (it's also the only thing that sets it apart from its chemically identical relative, the sapphire). The best are a true, vivid red, often called "pigeon's blood," which can fetch higher prices than any other colored gem.
First mined in ancient Egypt, the most valuable emeralds today come from Colombia and are highly transparent and pure green. Because the stones are naturally brittle and full of fissures, the rectangular-shaped (and aptly named) emerald cut is favored.
Distinguished by their hardness, sapphires come in a spectrum of colors, including pink, yellow, green, and salmon (called padparadscha). The most precious are from Kashmir, rendered in a rich violet blue that appears ultra-bright even in evening light.
Often referred to as "rock crystal," this clear, colorless variety of quartz was used to create the shimmering chandeliers at the Palace of Versailles. While large, flawless pieces are rare, smaller varieties cost less than $100.
Available in an unparalleled range of hues, they often feature two or more in a single cut. The rarest are neon blue or blue green Brazilian Paraiba tourmalines, which can command $10,000 per carat. But green and pink "watermelon" combos are currently having a fashion moment.
The "king of gemstones" is the most widely desired and generally the most expensive stone on the market. Clear white diamonds are prized, but red, green, pink, purple, and blue are rarer, and can sell for more than a million dollars per carat.
Hailing almost exclusively from Australia, these distinctively patterned stones can be found in a wide variety of hues, but black opals with a bright "play-of-color" (that is, the way its colors change as it's rotated) are the most valuable, sometimes netting $10,000 a carat.
Once obtained only by professional divers, the vast majority of the shiny orbs—which come in colors ranging from white to peacock green—are now cultured in controlled environments, making them much more affordable and widely available.
Now sure how to tell if your gems are conflict-free?
For many, precious stones are a symbol of love. For others, however, they are a by-product of violence, labor exploitation, and environmental harm, as some gems have been mined in part to finance civil wars and repressive regimes around the globe. Unfortunately, because the supply chain is so incredibly long, the only way to guarantee what you're buying is conflict-free is to opt for a rock grown in a lab (for diamonds, California-based Diamond Foundry is a sound option). That said, many companies are doing their utmost to ensure their goods are on the up-and-up. Go with a dealer you trust, and be sure to ask for detailed information about where each stone was mined, as some countries have better records than others. Diamonds from Botswana, Canada, Namibia, Russia, and South Africa are safer bets, as are sapphires from Australia, Malawi, and Sri Lanka, emeralds from Colombia and Zambia, and rubies from Malawi and Tanzania.
And.....81 Billion is the amount (in U.S. dollars) spent on diamond jewelry worldwide in 2014 (Source: De Beers Group)