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the first C, of the 4 C's carat weight :

the round brilliant cut :

 hearts and arrows :

the third c of the 4 C's clarity :

the forth c of the 4 C's colour :

an overview of coloured gemstones :

identification of diamond and simulants :

diamond manufacturing  process :

precautions during manufacturing :

weight estimations :

role of  laboratories in the diamond industry :

the latest technology in the diamond industry :

laser processing in diamond manufacturing :

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Optical Properties of Diamonds

The beauty and sparkle scintillation of diamonds depend on light. Facets create the magic with light due to optical effects and bring out the natural properties of diamond. That is why, it is very crucial to understand the optical properties of the diamonds. The single most important optical property observed in transparent minerals is the Index of Refraction. Each mineral has a distinctive index of refraction, or a distinctive range of indices. Also, most transparent materials have slightly different indices of refraction for different wavelengths of light, so light tends to be dispersed (figure 6.1). Among well-known minerals, values for the index of refraction range from opal 1.4 to diamond 2.42. The index of refraction for the violet end of the spectrum is greater than for the red end. This means that when white light is refracted, the violet portion is refracted more than the red. This is why a cut prism placed in the path of white light produces a beautiful display of the visible spectrum. Dispersion is recorded as a value equal to the difference between the index of refraction for red and the index of refraction for violet. Dispersion ranges from a low in fluorite (0.0087) to a high in diamond (0.044).

Optical Data of Diamond

Type                        : Isotropic

RI values                 : na=2.418

Max. Birefringence : o=2.418-lsotropic minerals have no birefringence

Surface Relief          : Moderate

Dispersion

Diamonds most precious treasure of nature gifted to man. It is also considered to be a best friend, specially for ladies. Till the beginning of 20th century it was very little understood, but for a very long time scientists were not interested in the glitter of diamond. Later on the mysteries of diamond were disclosed one by one by scientists. Dispersion is among one of them. Natural diamond has specific characteristics to disperse the white light into its spectral colours purple, blue, red, green, yellow, orange. Dispersion is a process which takes place in diamond and as a result what is it seen to the human eye is called Fire. Diamond's dispersion is 0.044, Green Garnet 0.057, Synthetic Zirconia's 0.065, Zirconia's 0.039 and Emerald 0.014.

Refraction

Refraction is very easy to understand. Generally it means to bend or to curve. When an object is kept in two media then it looks different in the two media. Suppose we keep a stick in a bucket full of water then it will look curved in water, straight outside water. Another example is of ray of light. When it goes to dense medium from air medium, then it becomes slow and the direction of ray is also changed. That changing of direction is called refraction. When the ray of light goes inside the dense medium then it curves towards perpendicular and when it moves out from dense medium to air medium then it moves away from perpendicular. We can better understand the refraction by figure 6.4.

Figure 6.4 : Angle of refraction (refraction of light in a diamond)

Figure 6.5 : Angles of Incidence, reflection and refraction                                                                                                                                       Figure 6.6: Refraction and Dispersion of light in a diamond

Refractive index of a diamond:

The speed. of light in air medium is 3,00,000 km/sec and it is same in vacuum. And the speed of light in diamond is 1,24,120 km/second. So if we do the process then it will be like this.

Refractive Index = speed of light in air medium = 300000 = 2 417

of Diamond                  speed of light in diamond  124120

So the refractive index of a diamond is = 2.417

Refractive Index of different elements

Table 6. 1 : The refractive index compares the velocity of light in a substance to that in vacuum. Diamond shows light to a remarkable degree, and ranks high in refractive index.

The Critical Angle of a diamond:

The critical angle of diamond is 24.41°. What happens in this: when we change the angle of incidence ray, the angle of refraction is also changed. And a time comes when the angle of refraction becomes of 90°, at that time what the angle of incidence of the remains is called critical angle. And the critical angle of a diamond is 24.41 0. That means that critical angle and the angle of refraction are co related.

The CRITICAL ANGLE

FigureS.7: The CRITICAL ANGLE of Diamond is 24.410 to the normal (imaginary line perpendicular to the interface of diamond and air.)

Total Internal Reflection In diamond (TIR)

Total internal reflection is a phenomenon which takes place in a diamond. This phenomenon makes a diamond beautiful. The appearance of a diamond depends on its total internal reflection. When we suppose 100% light falls on diamond, then and there 17% light reflects from the surface, and 83% light penetrates the surface and goes inside.

Now it is the cut of a diamond which brings out the 83% light. So now suppose 83% as 100% then it is the cut which brings out 98% light. That's why the distribution of parameters is very important in fashioning a diamond. And that's why the setting of angle is so important. If we take a good proportion then, and we set the angle perfectly then the incident of (TlR) happens. Its process is somewhat like this, if we increase the angle of incidence than its critical angle, in that position the rays of incidence do not refract in the dense medium, that travels up to the inner surface of bottom and reflects the light to the parallel opposite bottom facets and those opposite bottom facets reflect the light outside .This process is called the total internal reflection of a diamond. According to gemmologists if we coat the bottom with black paint still there will not be any bad effect on the appearance of the diamond. There is such type of setting of angles of diamond which does not allow the rays to move outside the diamond.

Scintillation, Brilliance and Fire:

Brilliance, Fire and Scintillation or Sparkle are the three factors of diamond's light performance which makes a diamond beautiful. For the three optical properties, good symmetry and good polishing along with an ideal proportion are a must.

Scintillation:

Scintillation is the most vital characteristic of a good cut diamond. Scintillation is generally occurring in diamond in two cases. The first one is, there should be movement in the diamond if the light is fixed. In that case scintillation in the diamond emits small particles of colourful light in the surroundings. It is also known as sparkling. Scintillation appears only when either diamond or source light is in movement. We can see the scintillation only if we are very attentive to it.

Figure 6.9: Microscopic View of diamond Figure 6, 10: scintillation is the flash of brilliance

showing journey of light  and fire that happens when diamond responsible for scintillation   

Fire:

"Fire is the visible extent of light dispersed into spectral colours". Colour light of a diamond is commonly referred to as Fire. Fire is an outcome of the process that is called Dispersion. The brilliance scope analyser measures the percentage of the diamond returning the Spectrum or the constituent colours of white light to the observer.

Brilliance:

Brilliance makes the diamond beautiful and is made of two necessary components brightness and contrast. Bright diamonds return lots of light from the surroundings back to a 'face up' an observer. If light from above leaks out the back of a diamond, naturally it has less brightness. But light that enters and leaves in the face up direction is wasted because your head blocks lights from that direction. Diamonds that are too deep or very shallow do this - they have areas that act like a mirror back to the viewer; they return less light and so they have less brightness. But to be brilliant, a diamond needs more than just brightness from light return. Consider the contrast of a chess board, although it has only 1/2 the light return of a sheet of white paper, it appears brighter, especially when it is moved because it 'scintillates'.

Figure 6. 12: A moving chess board appears 'brighter' than a sheet of white paper that has only half the light return

Behaviour of light in a well cut diamond, deep cut, shallow cut, too deep cut and too shallow cut diamond :

A well cut or an Ideal Cut diamond gives most brilliant light, while deep cut or shallow cut diamond reflects medium light, but a too deep or too shallow diamond shows very poor brilliance. Total internal reflection (TIR) is necessary to bring out the light from the crown side. Total Internal Reflection depends upon a diamond's parameters distribution or proportion setting. Differences among different cut diamond can be easily understood from the following Figures 6.13.

Fluorescence :

Fluorescence the word is taken from "Fluorspar" which is derived from" Fluor", a Latin word and "Spar" means a number of things arriving suddenly at the same time. Fluorescence is also known as "Luminescence". Mostly fluorescence is found in blue and purple colour (near about 98 %) but 2 % diamonds have yellow, green and pink colour fluorescence, which is very rare in existence. To measure the intensity of fluorescence, a machine is developed known as an UV (Ultra Violet) Colorimeter.

Appearance of fluorescence colours in diamond:

In diamond generally fluorescence appears in two shades. The first and most common shade found, is blue and a or blue purple mix. The other shades which are more rare and found in diamond are yellow and pink. In exceptional white or rare white diamonds like D-E-F-G-H where fluorescence occurs then the diamond goes towards minus point of the colour scale and in colour shaded diamonds Like I-J-K-L colour diamond for example then the colour moves toward the plus point of the colour scale, it would be likely that there is a chance 4 % price +/- the value of this type of diamond.

General degrees of grading diamonds regarding fluorescence

Generally there are five degrees of diamond fluorescence:

The difference between Fluorescent and Phosphorescent diamonds:

A magnificent property of some diamonds is that they can glow in the dark. When illuminated by ultraviolet light , certain diamonds can absorbs the high energy radiation and re- emit it as visible light. These diamonds are called Fluorescent. Some can even continue glowing after the ultraviolet source is turned off. These diamonds are called Phosphorescent.

Two optical responses of a fashioned diamond

There are two types of optical responses of a diamond:

                    Positive Responses                          Negative Responses

               (A) Scintillation                     A) Fish- Eye

               (B) Brilliance                           B) Nail-Head

               (C) Fire                                   C) Light-Leakage

               (D) Contrast

Fish-Eye and Nail Head

Fish Eye:

Fish Eye is a nasty appearance in the diamond that you can see just inside the table of a diamond. It looks like a dead fish eye. The fish eye is the reflection of the girdle. If the girdle is not polished and is thick, the effect looks like a Big Circular inclusion and can be as bad as an I-3 (PQ3). 41(degree) Pavilion and 72.2% table 39 (degree) Pavilion and 58.4% table. Diamonds with these proportions show a Fish Eye that requires no tilt to see the imperfection, if the table is increased by as little as 1% you can see 1% more fish eye.

Nail Head

The Nail Head effect in A polished Diamond comes about when the pavilion is cut so deep the the head of the diamond looks dark to the viewer from the top, The best way to illustrate the need for incorporating the effect of the viewers physical presence on the brilliance is a diamond with pavilion and main facets between 43 and 45 degrees. The GIA states that "If the Pavilion is very deep, much of the light is leaking out of the stone, then the star facets look almost black, and the stone is called a nail head.

Factors that determine the visual appearance of a diamond:

1.     The Source of Illumination :

1.1    Radiation Spectrum

1.2    Luminance

1.3    Angular Size

1.4    Source Position

1.5    Quality of Source

1.6    Colour of Surrounds

2.     Optical Effects of Diamond:

2.1     Brilliance

2.2     Scintillation

2.3     Fire

2.4     Spatial Contrast (static)

2.5     Temporal contrast (dynamic)

2.6     Light return

2.7     Leakage

2.8     Spread

2.9     Nailhead

2.10   Fish Eye

2.11   Culet seen through crown bezels

2.12   Windowing (see through)

2.13   Transparency of material

2.14    Polish

2.15    Colour

2.16    Clarity

3. Viewing conditions:

3.1      Pupil Adaptations

3.2     Observer position

3.3     Stereoscopic Vision

3.4     Psycho Physiology

3.5     Stereo type and market fashion

3.6     Pupil Motion

3.7     Colour Perception

3.8     Background Contrast

3.9     Eye's resolution ( spatial and dynamic)

3.10   Eye's sensitivity