NGTC Diamond Grading
DFHK / Diamond w/ Confidence / NGTC Diamond Grading
NGTC Diamond Grading 鑽石鑑定

Diamond Testing
Hong Kong has a simple and free business system and it does not enforce a compulsory grading certification for jewellery. If it does not come with a certificate and if in doubt you may request the retailer to refer you to or to submit the diamond to a reliable and accredited laboratory for a grading report. You may request a same day service if needed, be sure to ask for a quotation for grading charges beforehand.

In Hong Kong, DFHK and its collaborate organization, NGTC(HK), provides diamond and jewellery grading as small as 20 points. All diamond grading will go thru an authenticity check if it is a natural untreated diamond before proceeding with the quality grading. A report number may be laser inscribed on the girdle of the diamond. Matching of the diamond with the report becomes easy; consumer may easily verify the information on-line. With this the consumer can have an added assurance on the quality of diamond.

Many international brands present in Hong Kong, such as Gemological Institute of America (GIA), International Gemological Institute (IGI) or Diamond High Council (HRD).

NGTC Introduction

The National Gemstone Testing Center (NGTC) is a state-level gems, jewelry and jade quality testing institution in China. It was officially founded in 1992, headquartered in Beijing and has established top-ranking branch laboratories in Beijing, Shenzhen, Shanghai, Guangzhou, Yunnan, Shenyang, Pingzhou and Hong Kong with state-of-the-art instrument and skilled staff. It is China’s biggest and most sophisticated institution in gems and jewelry quality inspection.

Founded in 2009, NGTC (HK) laboratory equipped with professional instruments and technical staffs. It has passed the qualification certification and laboratory accreditation of ISO/IEC 17025 and relevant requirement of CNAS. It is an authoritative testing institution with qualification in gems identification, diamond grading and precious metal testing acknowledged world widely.

NGTC(HK)provide diamond and jewelry testing and grading service and issue identification and grading certificates for companies and consumers in HK. All diamond grading will go thru an authenticity check if it is a natural untreated diamond before proceeding with the quality grading. A report number may be laser inscribed on the girdle of the diamond. Matching of the diamond with the report becomes easy; consumer may easily verify the information on-line. With this the consumer can have an added assurance on the quality of diamond.

China National Accreditation Service for Conformity Assessment (CNAS) under the Chinese Government evaluate laboratories’ technical capabilities in accordance with the ISO / IEC 17025 standard, and issue accreditation to qualified laboratories. Accredited laboratories perform the follow eight test stipulated according to the National standards  and  established by the Standardization Administration of China:

1. Authenticity Identification
2. Measurement of Weight
3. Shape& Cutting Style
4. Colour Grading
5. Testing for Fluorescence
6. Clarity Grading
7. Symmetry and Polish Grading
8. Proportion and Cut Grading of Round Brilliant Diamond


4C’s Grading:

1. Diamond Color

The color of the diamond is rich and colorful, considering the output, grading method, value and other factors of the diamond of various colors, it can be divided into two series: one is colorless- light yellow (gray, brown) color series, also known as Cape Series; the second is the color series.

For the color of the colorless to light yellow series of diamonds can be expressed from high to low as D—N and less than N, in 12 consecutive grades, each letter represents a range of colors, D color diamonds are colorless, clear and transparent, and for diamond below N color grade have a clear yellowish color to the naked eye. The higher the level, the whiter the color and the higher the value. The vast majority of diamonds will have a slight yellow tone or gray, brown tone, so colorless and transparent diamonds are rarer and more expensive.

Professionals engaged in diamond grading use the most intuitive visual colorimetry method, comparing color of the diamond to be tested between adjacent grades of master stones to conclude on the result. The entire color grading process is carried out in an indoor environment without direct sunlight, and the surrounding environment is white or gray. Two to three technicians independently completed the color grading of the same sample, and obtained uniform results.


2. Diamond Fluorescence

Fluorescence means that a diamond emits visible light when exposed to ultraviolet light. 90% of diamonds are fluorescent, with different colors, different intensities, blue, yellow, white or other colors, and the degree can be divided into four levels: "strong", "medium", "faint", and "none". Similar to the color grading method, the most intuitive visual method is used to determine the fluorescence level. The diamond to be tested is compared with a set of natural diamond samples with a calibrated fluorescence intensity level under an ultraviolet fluorescent lamp. There are 3 samples in total, each of which represents the lower limit of the fluorescence level.


3. Diamond Clarity

The clarity features of a diamond include the internal and external features of the diamond, such as common internal features such as crystalline inclusions, feathers, etc., and commo external features such as surface texture. The less obvious the inclusions, the higher the clarity grade and the value. The clarity level of the National Standard can be divided into 5 major levels and 11 minor levels.

Professionals will observe the internal and external features of the diamond under a ten-fold magnifying glass and microscope, then record these features in drawings. The red mark represents the internal feature, and the green mark represents the external feature. The clarity of each natural diamond is different. This record is crucial as it help appraisers to grade the diamond and reflect the uniqueness of the diamond effectively.


4. Diamond Cut

For hundreds of years, numerous diamond cuts were developed and most of the designs aims at maximizing and preserving the original quality on top of meeting consumer’s demand for innovation. The most popular cut is the Round Brilliant Cut that best reflects the optical properties of a diamond. It composes of 57 or 58 facets. During the designing and cutting stage, the proportions and angles of each part are the critical consideration to ensure that the light enters the diamond and reflects multiple times inside the diamond to maximize the brilliancy. Simply put, a diamond rely on the best cutting ratio and angle combination of different parts to bring out the ultimate brightness, fire (dispersion) and sparkle.

How do the laboratory technicians evaluate the brightness, fire and sparkle of diamonds accurately and scientifically? Firstly, an overall observation of appearance of the diamond base on the above three aspects, which will be referenced at the later stage of determining grading results. Secondly, with the support of accurate data base, technicians will execute the three- dimensional measurement and multiple analytic function of the computer software. Furthermore, examine and evaluate the symmetry and polishing of the diamond under the microscope. Finally, various parameters and observation results are comprehensively analyzed, and the cut grade of the diamond is determined according to the National Diamond Grading Standard.

In terms of specific test items, cut is a comprehensive manifestation of ratio level and finish level, which can be divided into five levels: excellent, very good, good, fair and poor. The ratio level is determined by the cutting ratio, overweight ratio, brushing and polishing, of which the cutting ratio is the main factor, and the overweight ratio, brushing and polishing are the influencing factors. The degree of modification is determined by the symmetry classification and the polishing classification.


5. Diamond Weight

The weight of the diamond. The legal unit of measurement for diamond weight is grams. The most commonly used gemstone mass unit in the world is carat, which is also the mass unit commonly used for diamonds, and is derived from the seeds of the Mediterranean acacia tree. Because the weight of each seed is very similar, about 0.205 grams, it was often used as a weight for weighing diamonds in ancient times, but the carat weight of the old system was not uniform. It was not until 1914 that the jewelry world finally formed 1 Carat is the standard of 200 mg.

When the color, clarity and cut grade of the diamond are the same or similar, the larger the weight of the diamond, the higher the value. The final value of the diamond depends on all 4C grades.

To measure the weight of a loose diamond, the technician will weight it with an electronic scale with reading up to four decimal places and its accuracy can be achieved up to one hundred thousandths. For the weight of the diamond jewelry which has been inlaid, the reading will also be kept to four decimal places, and the accuracy will reach one ten-thousandth.