Gemologists value gems according to their rarity and beauty primarily. If they are sufficiently immersed in the buying and selling side of the business, they will know what a given quality of a gem is trading for at that time.
The name for a gem expert is a gemologist. They are experts in natural and artificial gems.
Whatever you have to sell is worth whatever someone will pay you for it. Best practices dictate that you take your gem to a certified gemologist who can document the characteristics of the gem. It's carat weight, its clarity, colour and cut. This professional evaluation will determine whether or not the gem has been treated or otherwise altered from its natural state, which would devalue the gem. A natural black diamond of this size will be exceedingly rare and costly. You may own a carbonado gem, which would be more common and less valuable.
You can read more about the diamond company you reference by following the link, below. As to a specific gem, you can take the gem to a certified gemologist to pay for more information about it.
To determine if a gem is original or synthetic, you can consult a reputable gemologist or have the gem authenticated through gemological testing. Characteristics such as color, clarity, and inclusions can indicate whether a gem is natural or synthetic. Advanced gemological tools and tests, such as spectroscopy and microscopy, can provide conclusive evidence about the gem's origin.
The raw stone's value will be what you're willing to accept from a gemologist or diamond cutter, who will cut and calibrate the stone as to its clarity and colour, and then sell it at a profit. A certified gemologist may be able to give you a value number. Actually, though, it's worth whatever you can sell it for. Educate yourself as to its potential colour and clarity though, before you attempt to sell it.
A single diamond weighing 38 carats will be more valuable than a collection totaling 38 carats. Every diamond is valued by its colour, clarity, cut and carat weight. Without a gemologist's certificate for each gem -- which you can pay for -- your diamonds are worth whatever someone will pay you for them.
Carat is the weight measurement for a diamond; a certified gemologist can give you an exact weight. Your local jeweler may be able to estimate the weight of the diamond by measuring its mm dimensions -- especially if you do not want the gem removed from its setting.
Whatever you have to sell is worth whatever someone will pay you for it. If you have a single stone of this carat weight, take it to a certified gemologist so that you can most fully understand its clarity, colour and the cut. This will help you establish a market value for it. If these are loose, gem-quality diamonds, you can have them documented individually. If you have a bag of industrial diamonds weighing ten carats, which are valued differently from gem-quality diamonds, the value will be determined in pounds not carats.
There is nothing called a "gold" "gem". Something is either made of gold or it is a gem, but it is not both.
Call some local jewelry stores and ask if they have a certified gemologist on site. If they do, take the stone for them to assay and they will be able to tell you.
Notice is a confusing word to use in this context -- perhaps you mean identify.A gemologist can help you determine whether your diamond is lab-created or a natural gem.
A gem expert is commonly referred to as a gemologist. These professionals are trained in assessing the quality, authenticity, and value of gemstones using various tools and techniques.