Basics of Faceting
In the gemological terms, facets are small polished planes arranged in a geometric pattern on a gemstone. Faceting is an art of cutting a faceted gemstone. It can be roughly divided into two aspects:
- Shaping - How facets are placed on a gemstone, and how a rough stone is cut into a certain shape.
- Polishing – After a stone is shaped, how facets are polished.
In this tutorial, these procedures are explained as well as a brief description of a faceting machine and a diagram.
Each facet is cut by grinding a rough stone on a flat round file called a lap. The lap turns around horizontally in a faceting machine. Usually water, oil or WD40 is used on the lap as a coolant/lubricant. The rough stone is adhered to a small stick called a dop. It is seated in the quill in a faceting head. The faceting head allows the dop and rough stone to swing horizontally at a set angle.
There are only three factors to place a facet:
- what angle to the dop to cut,
- which side of the stone to cut, and
- how much to cut off.
A faceting machine precisely controls these three factors with the faceting head. The angle control/protractor can be set the dop at certain angles. The index can rotate the quill and orient the stone to certain sides. The mast height controls how much to cut off.
Diagrams are blueprints of gemstone designs. They tell us to cut a rough stone at what angles on what indexes. However, a diagram does not show any direction as to how much to cut off. The individual faceter must decide how much to cut off. It depends on the design and the shape of the rough.
This is a diagram of the standard round brilliant cut. Reference: www.usfacetersguild.org/index_designs
To give you an idea of the steps a faceter has to follow, we are going to cut a round brilliant according to the design above. The material is dark red garnet. The diagram starts with the pavilion break facets. Let us set the angle control at 45 degrees and the index on 3. Adjust the mast height so the rough stone barely touches the lap. Grind little by little by sweeping the rough on the lap by hand. When the side on the index 3 is done, move to the index 51. Then move to 27and 75 and so on. Note that the diagram indicates the 96 index, and the interval of index numbers is 6. This means 16 equal facets are cut in a cone shape (96/6 = 16). When you finish cutting all around the rough at the same mast height, lower the mast height a little. Repeat this procedure until the rough looks like a cone. Check the progress with a 10 x jeweler’s loupe. All the facets should evenly come to a center point.
To cut the girdle, set the angle control at 90 degrees. Note that 90 degrees in the diagram means the dop is parallel to the lap. When you cut the table facet later on, you will see 0 degree in the diagram means the dop is perpendicular to the lap. The facet angles are always between 0 and 90 degrees. Adjust the index to one of the index numbers on the girdle facets in the diagram. Cut all around the rough until it forms a 16-sided short cylinder. This part will be the girdle and crown.
To place the pavilion main facets, set the angle control at 43 degrees and the index to 96. Bring the mast height so the rough stone barely touches the lap. Cut on the indexes 96-12-24-36-48-60-72-84. Note the interval is 12, which means you are cutting 8 equal facets instead of 16 (96/12 = 8). Lower the mast height little by little. Take a careful look at the pavilion figure of the diagram. Cut until the main facet tips meet the girdle as the figure shows. Take care not to overcut or undercut.
At this point, the pavilion facets and girdle need to be polished. These procedures will be described later in the Polishing section below.
To cut the crown facets, the stone needs to be transferred from the original dop to another dop on the exactly opposite side. For this stone, we use a cone dop. The cone dop accommodates the tip of the pavilion on a transfer block. Now you have the stone that has two dops on both sides. Remove the original dop by an appropriate method. Now the stone is transferred.
To shape the crown, let us start with the crown break facets. Seat the cone dop in the quill of the faceting head. Set the angle control at 47 degrees and the index on 3. Adjust the mast height so the stone touches the lap. Cut all the girdle facets until the girdle is approximately 0.5-1 mm wide. The girdle width should be proportional to the size of the stone.
Beginning of cutting the break facets. Notice that the break facets don't align perfectly with the girdle. This is not uncommon. There is an index micro-adjuster called a cheater on the machine. Use the cheater to correct the alignment.
Let us move on to the crown main facets. Set the angle at 42 degrees and the index to 96. Adjust the mast height so the stone barely touches the lap. Cut on the indexes 96-12-24-36-48-60-72-84. Note the interval is 12, which means you are cutting 8 equal facets. Cut until the tips of these facets meet the girdle.
Next we are going to cut the star facets. Set the angle control at 27 degrees and the index to 6. Adjust the mast height so the stone barely touches the lap. Cut on the indexes 6-18-30-42-54-66-78-90. The intervals are 12, but this time you are cutting the new facets in a staggered manner. Cut until the tips of the star facets meet the break and main facets.
At this point, the crown facets need to be polished. These procedures will be described later in the Polishing section below.
The diagram shows the angle of the table facet is 0 degree. The dop has to be positioned perpendicular to the lap. It can be done with a 45-degree table block. Set the angle control at 45 degrees. Engage the table block to the quill. Seat the dop in the table block. Cut the table by sweeping the stone.
Finally the table facet needs to be polished. These procedures will be described in the Polishing section.
You may have noticed that there is no directions as to what laps to use for shaping, or what agents to use for polishing in the diagram. The individual faceter must establish his/her own methods empirically, or based on information available from faceting classes, personal communication, publications, the internet etc. That is one of the reasons why faceting is considered an art.
The following pictures show a sequence of a crown being cut and polished at various stages. The material is YAG. It is an example to give you an idea how polishing goes. Faceters may use other methods and still obtain the same results.
For this particular stone, the pavilion and girdle have been already shaped and polished. The stone has been transferred to a cone dop. Since this is a large stone the bulk of the rough is removed with a coarse 260-grit lap. At this stage, pay attention to the alignment of the break facets to the girdle facets, i.e., cheating. Notice the wide girdle. Stop using the coarse lap when the girdle is 3-4 mm wide.
The crown is shaped on a 600-grit lap as explained in the Shaping section. Continue on cheating, and start paying attention to the meet points.
The crown facets are pre-polished on a tin alloy lap charged with 3,000-grit diamond. Pre-polishing goes through the same sequence using the same angles and index numbers in the diagram. Good pre-polish makes each facet surface ready to be polished. This stone shows a smooth, almost finished appearance.
Each facet is polished on a composite lap charged with aluminum oxide. Check the facet surface with a 10 x jeweler’s loupe to see if there is any scratch left on the facets. Polish the facets until they are completely smooth like a mirror. Polishing repeats the same faceting sequence in the diagram.
This is the finished stone off the dop and cleaned.
There are several choices of polishing methods depending on the types of stone. Most commonly used agents are fine grit diamond (50,000-grit or 100,000-grit) and oxides such as cerium oxide and aluminum oxide. They are used on a tin alloy lap or a composite lap.
Besides shaping and polishing, there are many more aspects on faceting. They include how to prepare and orient a rough stone, how to dop the rough stone, what adhesives to use for dopping, how to transfer the stone, and which laps to use at various stages for various stones etc. They are beyond the scope of this tutorial. You may find the information in the discussions on the GemologyOnline Forum or elsewhere.