Here's everything you should know before buying a diamond ring

Marriage is (hopefully) forever. And so is that big, sparkly rock you're about to spend a fortune on.

You've done the hard work of finding the right person; now here's how to find a diamond that'll outlast your love.

1. Do your homework.

First things first: familiarize yourself with the 4 C's (color, clarity, cut, carat weight), which will help you determine the true value of a stone.

Diamond color can range from yellow ("light") to colorless ("crystal clear"); the more colorless, the more valuable. Clarity is graded based on the presence of any nicks, scratches, or internal flaws.

Cut refers both to the amount of light the diamond refracts as a result of its proportions, and to the shape of the stone overall. Finally, carat weight is, well, weight. The average US engagement diamond is just under one carat, about the weight of a small paper clip.

Do your preliminary research with the Gemological Institute of America site, which is the gold standard of objective diamond information, according to diamond expert Chris del Gatto of the DEL GATTO online jewelry marketplace.

But don't think that reading a few pages of the site makes you qualified to spend the big bucks straightaway. "A little bit of knowledge will get you in trouble," del Gatto warns.

Be aware of what you don't know, too, and don't be afraid to request expert opinions or seek out further information.

Casey Sullivan, co-founder of online jewelry consignment shop Gleem & Co, agrees that the process can be "intimidating." But if you're smart about your approach, you can come away with something special.

2. Don't think of a diamond as just an investment.

"You're not buying real estate or Apple stock," del Gatto said. "Part of the return on this investment is the experience of seeing it every day."

"Get to where you can speak the language and know what you value."

Maybe color is more important to you; maybe you are more drawn to a specific cut. Recognize that beauty is, ultimately, in the eye of the beholder, and what you want will naturally be a subjective and personal preference.

The takeaway? Buy what you - and your significant other - love. And don't let anyone else tell you otherwise.

3. Do consider the shopping experience you want to have.

"The process can be as different as the person buying it," del Gatto said. You can go to a high-end retailer, like a Tiffany & Co., for that luxe experience. Sure, you'll pay through the nose for the gem, but if your fiancée wants the robin's-egg-blue box and a brand name, then nothing else will do.

Or you can stop at a big name like Kay or Zales, known for their diamond offerings and with a lower price point than the luxury department stores.

Another route is to look to online retailers like Blue Nile. As it's not brick-and-mortar, you can buy a diamond at a better price and more like a commodity, while still maintaining confidence that the quality will be backed by a reputable brand name.

4. Do check out the secondary market.

For the absolute most bang for your buck, secondhand diamonds are your best bet. Just because a diamond has been worn before doesn't mean it's any less beautiful or valuable. And the supply is huge, with - as del Gatto puts it - "zillions of dollars of product" sitting in safety deposit boxes or at auction houses.

DEL GATTO is an online marketplace for engagement rings and other jewelry at varying price points with a focus on engagement rings. Gleem & Co. hand-curates a selection of luxury jewels and watches, promises below-appraisal prices, and offers a personal concierge to help you find just the right piece.

On eBay, meanwhile, rings are also becoming a popular find: in the past 5 years, the engagement ring market has grown almost 60%, with an increase of 164% in vintage and estate engagement rings sold since 2008, an eBay spokesperson told us.

While traditional retailers will sell pieces at up to two times their appraised value, markets like these guarantee a much better cost, often at or below the appraisal.

5. Don't fall for a fake.

There are two tip-offs to watch out for. The first is weight: cubic zirconia, the "fake" diamonds often used in costume jewelry, are heavier than real ones. And second is color: a fake will send out more of a rainbow of refracted light than a real gem. But be warned that both of these signs require some serious background knowledge to spot.

"If it seems too good to be true, it usually is," del Gatto said. At the end of the day, diamond experts spend decades learning to spot fakes or otherwise low-quality diamonds. If you're ever in doubt, find someone to verify your find, Sullivan and del Gatto agree.

And it's also best to avoid working with anyone one-on-one. "You want to deal with a reputable company that has something to lose," del Gatto said, instead of with a small or private outfit. Retailers will guarantee their stones, while auction houses and marketplaces will provide an expert opinion and appraisal documents to allay your concerns.

6. Do spend on the stone, first and foremost.

Jewelry trends change all the time, but the diamond itself is going to last forever. Once you establish a budget, put all your money into the diamond and not the design, suggests del Gatto.

"The diamond won't go in and out of style, although the mounting will," he said. You can always change the setting, but you'll be happier with better quality in your stone. When boredom strikes or your taste changes - which often happens over the years - you can always have it reset, knowing that the core piece is still top-notch.

Because at the end of the day, the nicer the diamond, the more it will be appreciated for years to come.

Gleem & Co.

Yellow diamonds are also very popular, as seen on celebrities like Olivia Palermo. This ring is priced at over $100,000, and the central game is more than 6 carats.

7. Do consider larger trends in design.

"Millennials want things more personalized and more unique to reflect who they are," Sullivan said. "It's not as brand-driven."

When it comes to trends in stones themselves, del Gatto says he's noted a definite increase in the popularity of oval-shaped cuts in recent years. Sullivan also suggests that a pear-shaped diamond is a smart move: the cut creates the illusion of being much bigger than it really is, and hides imperfections well.

Marquise shapes are also coming into vogue again; their last heyday was back in the 1920s. In fact, vintage is back in vogue, with eBay reporting a 200% increase in popularity over the last five years, according to a representative of the online auction site.

For mountings, a "halo" effect - tiny stones all around the main diamond - is having a moment, while the baguette style (in which two more rectangular diamonds decorate the band on both sides of a central gem) is fading out, according to del Gatto.

Gleem & Co. is seeing more alternative and estate pieces involving additions like baroque pearls and colored stones finding popularity with buyers. The classic three-stone setting, meanwhile, is going out of style.

8. Don't wing it.

Be an informed buyer - and a prepared proposer. You'll be asked how you popped the question for many years to come, so a little planning is worth it.

If you're worried about the bulky ring box getting in the way of your picture-perfect plan, a lay-flat ring box to fit in your pocket might be an easy solution.

Otherwise, think of somewhere you'll both be comfortable, and take it from there.