Everything you need to know before buying an engagement ring

Diamonds are graded and priced based on the "four C's," which include cut, carat, color, and clarity.

The four C's are important to know because they help you understand the quality of the diamond, and they also help determine its price.

For example, a one carat round shaped diamond with an "ideal" cut grade can range from $2,521 to $12,857 at Ritani depending on its grade for clarity and color.

Cut grade determines the diamond's "sparkle" effect.

The cut grade is determined by the diamond's proportions and symmetry of each facet of the diamond — which directly effects the way the diamond captures and reflects light, creating that beloved "sparkle" effect.

Cut grade is measured on a scale from "ideal" to "poor."

Carat, which is the diamond's weight, doesn't determine the exact size of a diamond.

Because carat refers to a diamond's weight, two diamonds of the same carat weight might actually be slightly different sizes. Because carat weight effects the price the most out of the four C's, Ritani suggests shopping for a carat just slightly below the size you initially want — because the difference in size will be negligible to the human eye, but the price can be greatly affected.

The more colorless a white diamond is, the more rare and valuable it's likely to be.

Even white diamonds can have some color, usually a yellow or brown tone, depending on their grade. On a scale of D to Z, D being completely colorless, Z being a yellowish brown diamond, Ritani customers usually purchase a F to I diamond.

Ritani suggests setting the diamond in a gold band if you're looking to save money.

Clarity grade can be affected by the diamond's shape.

Natural imperfections that occur when the diamond is first created determine the clarity grade. The more clear a diamond, the most expensive it is.

The diamond's shape can also effect clarity — for example, round and princess cut diamonds that have many facets can hide certain imperfections. Emerald and asscher-cuts have a large, open-table shape that makes it easier to spot imperfections.

The most popular selling diamond shapes are round, princess, and cushion.

The shape of the diamond is extremely important to the overall style and look of the ring. "Millennials still want that timeless, classic look — just like any generation does," said Keeney.

According to Ritani, the three most popular shapes are the classic round (pictured above), a princess cut, which has a square shape, and a cushion, which is a square cut with rounded corners, creating a somewhat oval look.

Halo is by far the most popular setting.

The halo setting surrounds the main diamond with other smaller ones — increasing the overall size of the ring, and making the center diamond appear larger. The cushion shaped diamond in a halo setting is one of Ritani's best selling.

Remember, it's about her style.

At the end of the day, it doesn't matter what's most popular — it's all about what fits your budget and her wants.

According to Keeney and Ritani's market research, "She's after maximum sparkle, and she wants [the ring] to be a reflection of her personality, and be unique amongst her friends," he said.