A diamond is valued by its cut, clarity, colour and carat weight, which you state at four carats.
Depending on where you buy it, and how much documentation you get with your purchase, you could spend several tens of thousands of dollars or more for a diamond of this size.
Here are two examples from Blue Nile:
In addition, then, you'd pay for the ring and setting and any additional diamonds you'd want to add to the ring.
A 4.03 round D color, VVS1 clarity diamond costs about $240,995. Depending on your budget, you can adjust the shape, color, clarity and cut to get a price that fits within your budget.
A 4 carat diamond ring would be ridiculously expensive. In the $100K mark I would expect. If someone is showing you their 4 carat engagement ring, it is probably cubic zirconia.
1 carat can be up to $22,000. Diamonds can be up to $1,000,000 or more. The price varies greatly, and depends mostly on these four specifications: cut, clarity, colour and carat (weight of a jewel).
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/
For a complete answer, you must go to this site:RarityOf the 4 C's, Carat Weight is probably one of the most important considerations with respect to rarity, value, and consumer preference. In fact, Carat Weight, or how much the diamond weighs is the most significant value factor in a diamond. Because diamonds by nature are rare, a 2 carat diamond is actually about 3 times more valuable than a comparable quality 1 carat size. Statistically a 1 carat is one in a million diamonds mined, and a 2 carat is one in five million diamonds mined. Therefore, all things considered a 2 carat diamond is really "value priced" based on the rarity factor.PricingLarger diamonds are much more rare, thus commanding a higher price/per/carat. Carat weight affects the value of a diamond by about 10-20% for each step in size difference. Also, "magic sizes" refer to major carat weight categories, for example exactly 1.00 carat, or larger, and can prompt "price premiums". To calculate the price of a diamond you multiply the carat weight by the price/per/carat. For example: if a 1.25 carat diamond was priced at $10,000 per/carat the diamond would be sold for $12,500.* http://www.diamonds.com/education/Carat.aspx
The 25 inside your ring could be 2.5 gms or .25 (dot may be not visible) or 2.5. If the ring is made in Japan, 25 is ring size number. It may also mean that your diamond is 1/4 carat. http://www.diamonds-are-forever.org.uk/carat-weight.htm
It all depends on what we call the 4 c's clarity,colour, carat and cut. Your twenty carat stone might only be of industrial quality and then it depends if its still in its rough or uncut form giving a random price with diamonds is impossible
If the diamond is cut, 4-point may refer to its weight, would would otherwise be expressed as .04 carat, or 4/100ths of a carat.
1/4 carat or smaller.
A carat, in relation to diamonds, is a unite of mass that is equal to 0.2 grams. Meaning a 4 carat diamond weighs just over .8 grams and an 8 carat diamond is 1.6 grams.
A diamond is valued by its cut, clarity, carat weight and colour.
Best practices dictate that with a stone of this size, you hire a certified gemologist to document the 4 Cs for this diamond, and test the metal of the ring and include it in the certificate. Then you will have a base of reference for its worth. Regardless of its age, previous purchase price and other points of history about a diamond, each stone is valued by its carat weight, its cut, its colour and its clarity. The exception to this guideline is documented provenance: if a piece of jewelery or a diamond has been owned by royalty, a famous person, even a despot, its value may be enhanced.
The four C's are Carat, color , cut and clarity. Whenever you purchase any diamond always keep these 4 C's in mind,Carat is related to the weight of the diamond.Cut is related to the shape and design of the diamondColor is related to the color, color I mean be careful , science has found ways to add color to a diamondClarity is related to genuineness of the diamond. You can use a diamond Loupe as tool to measure a diamonds clarity.