Emerald is gem quality version of a mineral called Beryl. Beryl is a mineral composed of beryllium aluminum cyclosilicate with the chemical formula Be_₃Al_₂(SiO_₃)_₆. Well-known varieties of beryl include emerald and aquamarine.

picture of emerald

Beryl forms hexagonal crystals. Gem quality of a mineral means that the crystal is well formed morphologically, has no or minor impurities and no mechanical flaws or cracks.

That's more economics than science, since the jewelry industry inflates the price of emerald.

Emerald is a type of beryl, Be_₃Al_₂(SiO_₃)_₆, and there are several other varieties including aquamarine and others. It's rare, and it's pretty, which is why it's usually treated as a gemstone.>

Still, the price is basically how much someone is willing to pay to buy it.

The simplest thing that would make an emerald more valuable is its size. Bigger emeralds cost more than smaller ones of the same quality. The green color of an emerald is important: minerals that have the same structure are called beryl. A beryl can be many different colors. Some emeralds that are blue-green are also popular. I personally prefer those. Darker emeralds usually don't show the vibrant green as much, so they usually don't cost as much. So people value clarity. No emerald is completely pure, but many emeralds have impurities which you can see with the naked eye. Gem cutters, the people who take the raw stone and cut it into the form seen in stores, make a big difference because they can cut gems in ways that display certain colors better or make the stone more durable. Really the most important idea behind all these ideas is to try to find a stone that looks pure, is large, and catches the eye. Its hard to find those kinds of emeralds, and that's why they're so valuable.

As with many other gemstones, the monetary value of emeralds increases with increases in size. Value also depends on color - the most desirable is slightly bluish green. Clarity, or how clear or cloudy the emerald looks, also matters. These traits are also valued more on an unchanged natural emerald than on a treated or otherwise changed emerald. However, one thing to keep in mind is that the reason gemstones cost money is that they look nice and are rare in nature, so rarity is an important factor in the determination of price.