Like many stones, the per carat price of fine quality emerald escalates rapidly with size. We found a Genuine Indian Tigris Emerald Loose Oval Cut Gemstone on sale at eBay for $4.99.
As explained by the International Gem Society, value factors hinge largely on color, with nuances of saturation and hue affecting price to a significant degree. The most desirable color is a slightly bluish green in a medium dark tone with strong saturation. Clarity is important, but inclusions are tolerated more in this variety than virtually any other gem. Top quality, unenhanced stones (with certification) can bring as much as 50% more in price than treated stones of the same size, color, and clarity.
You can see price range for Emeralds at SinghalGems.com.
Check out gem shopping tips at International Colored Gemstone Association.