The American Gem Society is a great place to start on getting an appraisal for a 3 carat diamond ring. They have independent certified Gemologist appraisers that are very skilled in rendering a valuation report for a diamond of any shape and size.
Take your ring to a local jeweler and pay for an appraisal. If the diamond is larger than one carat, you can pay a certified gemologist to document the characteristics of the diamond and verify the composition of the metal setting.
One diamond carat weights 0.007054792 ounce: you do the math.
The number inside a diamond ring refers to the carat weight of the diamonds found in the ring. A .50 stamped inside a Diamond Solitaire ring means that it has a 1/2 Carat diamond while a .97 means that it is just below 100 points or One Full Carat.
This indicator means that if all the diamonds in the ring were removed, their combined carat total weight, would be equal to one-half a carat.
Your local jeweler can show you a ring like this, and give you a price for the one you want.
A one carat diamond ring that costs $4,125 in 2013, sold for approximately $400 in the 1950s.
A one carat diamond weighs one carat and contains one carat.
A 2 carat diamond cushion cut ring would make a beautiful engagement ring. The price of such a ring would vary from jeweler to jeweler; however, one could expect to pay from around $450 upwards.
The price for a one carat diamond cluster will vary greatly depending on things such as the quality and size of the diamonds in the cluster. The type of ring setting is also a factor in price such as 10, 14, 18 or 24 carat gold. An example is a 10k white gold diamond cluster is $600 at Zales but a 14 carat diamond cluster on the Ross Simons site sells for $2,471.
Both versions of the description you give, 20 point and 0.2 carat mean that the stone is roughly one-fifth of a carat in weight.
A one carat diamond weighs one carat, or .20 grammes.
When buying diamonds you need to consider the 4 C's - Cut, Clarity, Color and Carat. The last one inform you of the size of the diamond. For more have a look at the following http://adiamondvault.com/article/diamond-carat-guide/