Diamond Grading

NGTC Hong Kong Laboratory provide services of the identification and garding of colorless and yellow diamonds according to the national standards GB / T 16553 , GB / T 16554  and GB / T 34543< Yellow Diamond Grading> issued by the Standardization Administration of the people's Republic of China. The laboratory will conduct the following tests on diamonds:

1. Authenticity identification

2. Weight

3. Cutting shape

4. Color grading

5. Fluorescence 

6. Clarity grading 

7. Symmetry and polishing grading

8. Ratio and cutting grading of round diamonds

4C grading of colorless series diamonds

The international 4C grading method is adopted by NGTC Laboratory for diamond grading, which is to grade the quality of diamonds from four aspects: carat, color, clarity and cutting.

Diamond color

The color of diamond is rich and colorful. Considering the diamond output, grading method and value of various colors, it can be divided into two series: one is colorless to light yellow (gray and brown) series, also known as Cape series; the other is color series. For the colorless - light yellow series of diamond color, from high to low can be expressed as D-N and less than N, 12 consecutive levels, each letter represents a color range, D-COLOR diamond colorless and transparent, no heterotone, below N is the level of the diamond naked eye visible obvious yellow. 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 transparent diamonds are very rare and expensive.

Professionals engaged in diamond grading adopt the most intuitive visual colorimetric method to compare the colors of the diamonds to be tested between the adjacent color master stones, so as to obtain the results. The whole color grading process is carried out in the indoor environment without direct sunlight, and the surrounding environment is white or gray. Two or three technicians independently completed the color grading of the same sample, and achieved unified results.

Diamond fluorescence

Fluorescence refers to the visible light emitted by a diamond under ultraviolet light. 90% of the diamonds are fluorescent, with different colors and intensities, including blue, yellow, white or other colors. The degree can be divided into "strong", "medium", "faint" and "None". The same as the color grading method, the most intuitive visual method is used to determine the fluorescence level. The diamond to be tested and a set of natural diamond samples with calibrated fluorescence intensity level are compared under the ultraviolet fluorescent lamp. There are 3 samples in total, and each sample represents the lower limit of the fluorescence level.

Diamond clarity grading

The features of diamond clarity include the internal and external features of the diamond, for example, the common internal features include crystal, cloud, pinpoint and so on, and the common external characteristics include surface texture and so on. The less obvious the inclusion content is, the higher the clarity grade and the higher the value of diamond will be. In the national standard, the clarity can be divided into 5 major grades and 11 detailed grades.

Professionals engaged in diamond clarity grading will observe the internal and external characteristics of diamonds under a 10 times magnifying loope and microscope, and draw and record these characteristics. The red mark represents the internal characteristics and the green mark represents the external characteristics. The clarity of each natural diamond is different. This record can not only effectively help to identificate and grade the diamond, but also reflect the uniqueness of the diamond, so it is very necessary.

Clarity Sketch:

Cutting Grade

For hundreds of years, people have designed hundreds of diamond cutting types, most of which are designed to maximize the original quality and meet the market demand for novelty. The standard round brilliant cut, or round brilliant cut, is the most popular diamond cut in the market. The standard round diamond is composed of 57 or 58 facets. The proportion and angle relationship of each part are the key points to be considered in the design and cutting. For example, how to ensure that the light can be seen by the observer to the greatest extent after being reflected into the diamond for many times? How to make diamonds look more dazzling and "fire color" more intense and so on. In short, diamond cutting depends on the best proportion and angle combination of different parts, so that the diamond shows the best brightness, fire color (dispersion) and flash.

So for the technicians in the laboratory, how to evaluate the brightness, fire color and flash of diamonds more accurately and scientifically? First of all, we need to observe the overall appearance of the diamond from the above three aspects, which can evaluate and correct the grading results in the later stage. Secondly, we need a lot of accurate data support. Technicians will check and test multiple items through the three-dimensional measurement function of computer software, and strictly evaluate the symmetry and polishing of the diamond under the microscope. Finally, according to the national standard of diamond grading, the cutting grade of the diamond is determined by comprehensive analysis of various parameters and observation results.

In terms of specific test items, cutting is the comprehensive embodiment of ratio level and decoration level, which can be divided into five levels: excellent, very good, good, fair and poor. The ratio level is determined by three factors: cutting ratio, overweight ratio, brushing and grinding, in which cutting ratio is the main factor, and overweight ratio, brushing and grinding and grinding are the influencing factors. The degree of decoration is determined by symmetry and polishing.

Diamond weight

The weight of a diamond. The legal unit of measurement for the weight of a diamond is gram. Carat is the most commonly used unit of gem quality in the world, which is also the common unit of diamond quality. It comes from the seeds of Sophora japonica. Because the weight of each seed is very similar, about 0.205 g, it was often used as a weight for weighing diamonds in ancient times, but the weight of the old carat is not uniform. It was not until 1914 that the jewelry industry finally formed the standard of 200 mg for 1 carat.

When the color, clarity and cutting 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.

When measuring, the technician will put the diamond in the electronic balance and press the button to display the reading. For the loose diamond, the reading accuracy will reach 1 / 100000, and the counting will be kept to four decimal places; for the diamond jewelry that has been inlaid, the counting will reach 1 / 10000, and the counting will be kept to three decimal places.

4C grading rules of colorless diamonds

Yellow diamond Grading

Besides the grading of colorless diamonds, the laboratory can also grade yellow series diamonds in color, clarity and carat, according to the National Standard GB/T34543. Compare to the traditional colorless diamond 4C garding system , the biggest difference lays in  the color grading method, while the classification method of clarity and carat is basically the same as that of colorless series diamonds. 

Diamond grading certificates

To meet the different needs of customers, the laboratory issue certificates of different types and sizes for colorless and yellow diamonds. The most commonly used are full 4C certificate, small 4C certificate, half-folded mounted diamond grading certificate, and diamond sorting label.

Identification of Colored Gemstones
The NGTC Hong Kong laboratory provides identification services and issues certificates for various common colored gemstones. All tests are carried out in
accordance with the National Standards of the People's Republic of China "GB/T
16552 Gems-Nomenclature" and "GB/T 16553 Gems-Testing".


Gemstones types:

Currently we provide testing service for gemstones as belows:




















Other commen gemstones.


Testing procedure:

The laboratory is equipped with professional gemologists, and diverse gemological testing instruments
and equipment, which can perform testing on the samples' quality, density, inclusion inspection, dichroism, refractive index,polarization, ultraviolet fluorescence and phosphorescence , luminescence structure observation, and various gemological identification features such as ultraviolet-visible spectra,
infrared spectra, elemental analysis, Raman spectra, etc. Our professional gemologists will conduct
comprehensive analysis of the testing data of the sample, and draw accurate and scientific gemological
conclusions. The naming and description of the appraisal results are in accordance with the requirements
of the national standard "GB/T 16552 Gems-Nomenclature" to avoid any misleading and confusion. If
the gem is found to be artificially synthesized, or has undergone any artificial treatment to improve its
appearance, we will make clear declaration on the certificate.

If the test sample is mounted gemstone jewelry, in addition to the identification of the gem itself, we can
also determine the type and content of the precious metal used in the jewelry piece, and we can also
identify the authenticity of the melee-diamonds or other side stones on the jewelry.

The detection process can be illustrated as follows:

Identification Certificates:

The test conclusion, weight, shape, color, magnified inspection features of the sample, as well as the
precious metal conclusions will all be disclosed on the certificate, accompanied by a photo of the sample
for identification. In addition, there are the signatures of the inspector and the approver, the date of
inspection, and the testing standard adopted. All certificates can be checked for authenticity on the
official website of the NGTC headquarters with a QR code or barcode or certificate number. The inquiry
address is www.ngtc.com.cn 

Below are certificate samples:



Feicui (Jadeite) Identification

Definition of Jadeite (Feicui):

A kind of granular to fiber-interlaced polycrystalline aggregate mineral, mainly  composed of jadeite or jadeite and other sodium, sodium calcium pyroxene (such as sodium chromium pyroxene, pyroxene),  may contain a small amount of hornblende, Feldspar, chromite and other minerals.

Basic mineral characteristics:


Chemistry composition:Jadeite( NaAlSi2O6), could  contain   elements such as   Cr、Fe、Ca、Mg、Mn、V、Ti  etc.

Crystalline state: crystalline aggregates, often fibrous, granular or partially columnar aggregates. Common colors: white, various shades of green, yellow, red orange, brown, gray, black, light purple, purple, blue, etc. Luster: Glass luster to grease luster.

Cleavage: Jadeite has two sets of complete cleavage, and the aggregate can see tiny cleavage surface flashes, which is called "greenness".

Mohs hardness: 6.5~7.


Optical characteristics: heterogeneous aggregates.

Polychromaticity: Unmeasurable.

Refractive index: 1.666~1.680 (±0.008), usually 1.66 by spot measurement.

Birefringence: The aggregate is unmeasurable.
Natural jadeite is commonly known as "Grade A" jadeite.

In addition to natural A jadeite, the laboratory often encounters other varieties of jade imitated jadeite, artificial jadeite, and enhanced or treated jadeite during the identification process. For these situations, the laboratory distinguishes them through scientific testing methods and makes clear indications on the certificate.

Instruments used:

Certificates samples:

Diamond Identification

NGTC (HK) lab provide diamond screening and identification service to loose diamonds and mounted diamonds in all size.

  • Identification methods

Multiple testing equipments are used in our lab by technical staffs to observe the luminescence images and collect spectra of diamonds. By testing and analyzing the data, we can accurately distinguish natural diamonds from synthetic and treated ones.



DS3000 diamond screening machine

  • To detect 415nm peak.
  • Natural diamonds are mostly with 415nm absorption peak, while synthetic diamonds do not.
  • When 415 nm peak is tested, result shows pass.
  • When 415nm peak is not tested, result shows refer, which means further test on FTIR is required.



FTIR spectroscopy 

  • To separate the diamond type into Ia/Ib/IIa/IIb.
  • Natural diamonds are usually type Ia, seldomly type IIa/Ib/IIb.
  • HPHT synthetic diamonds are normally type Ib/IIa/IIb.
  • CVD synthetic diamonds are normally type IIa.


DiamondView TM

  • To observe the fluorescence and phosphorescence of diamonds.
  • Natural diamond has different color and intensity of luminescence compared to synthetic. Moreover, the growth structure of natural and HPHT and CVD diamonds are totally distinguishable.


GV5000 and Synth-detect:

  • To observe phosphorescence of melee-sized diamonds quickly.
  • Normally natural diamonds don’t have phosphorescence while synthetic tend to have strong phosphorescence. Also in special case when synthetic don’t phosphoresce the fluorescence color are different from natural which can be another clue in quick screening of melee-diamonds.




  • Identification certificates:

Based on customers’ requirements we issue different types of certificates, among which the most popular ones are “half-fold” and “card” size.

Below are certificates samples:

Half-fold size:


Card size:


  • Identification conclusion:

According to the national standards , we draw the conclusion and description that shown on the certificate.


  • If the diamond is natural origin,

 the conclusion is : Diamond/mounted with diamond;

  • If the diamond is synthetic origin,

the conclusion is : Synthetic diamond;

  • If the main stone is natural but the side stones are synthetic,

the conclusion is : Diamond, mounted with synthetic diamonds;

  • If the diamond is natural origin but went through post-growth treatments such as High-pressure High –temperature/irradiation/filling/laser drilling to improve the appearance,

the conclusion is : Diamond (treated), remarks: the diamond is HPHT-treated (or etc).


  • Diamonds quick screening service—for Cluster setting diamonds jewelry and parcel melee-diamonds

We can do quick screening of melee-diamonds whether mounted or loose. Natural diamonds can pass the screening directly, and doubtable ones will be marked or picked out, for customer to do “further test” to verify the origin.

“Further test” means the diamond is possible to be synthetic but not sure, the lab will need more testing data to draw the final conclusion.

If client accept the additional time and fees arrangement regarding the further test, lab will then conduct more complicated and comprehensive testing and analyzing on the sample until the final conclusion is made. Normally it will take extra 2weeks.



Diamond Laser Inscription Service

NGTC(HK) Lab equipted with one Belgium-made high-precision cold laser inscrition machine PS500. Both NGTC logo and number of certificate can be inscripted on the girdle of diamond, servring as the unique identification mark of the diamond. 

Precious Metal Jewelry Testing

NGTC Hong Kong Laboratory provides precious metal purity testing and certification services for common platinum, gold, gold and silver jewelry. The detection is based on the national standards of the people's Republic of China "GB 11887 regulations and nomenclature of precious metal purity" and "GB / T 18043 determination of precious metal content by X-ray fluorescence spectrometry". The type and purity of precious metals were determined by nondestructive testing.

Precious metal mark:

Before the test, the inspector will first check the mark engraved on the precious metal sample.

The complete mark must contain both the type and purity of precious metals. For example: 

Au750 or G750 or G18K;
Au990 or 足金;
S925 or Ag925
Incomplete mark: lack of precious metal type or purity.For example: 

750 or 990 (only content, but no precious metal types).
Pt or Au (only type, but no purity)

Common precious metal types and purity expression methods:

Precious metal purity determination method:

The laboratory adopts non-destructive testing methods, and professional technicians use X-ray
spectrometer to determine the type and purity of precious metals.

Measurement principle: The elements on the surface of jewelry are excited by X-rays (the
thickness of penetration is usually a few micrometers to tens of micrometers), and emit
characteristic X-ray fluorescence spectra, which are qualitatively analyzed according to their
characteristic spectral lines (energy or wavelength). There is a certain linear relationship between
the X-ray fluorescence intensity of different elements and their content. As the content of the
element to be measured goes from low to high, this linear relationship goes from strong to weak,
and the calculation method gradually transitions from direct method to normalized method. By
comparing and calculating with the working curve of standard materials, quantitative analysis can
be carried out.

Factors affecting the measurement results: 1) There is a big difference in the element
composition and content between the tested sample and the standard material. 2) The surface of
the sample to be tested is coated or chemically treated. 3) The measurement time. 4) The shape
of the sample. 5) The measured area of the sample. 6) The content of precious metals. 7) The
uniformity of the sample to be tested (including segregation and soldering flux, etc.) .

Certificate samples:

Price List