How to Grade Onyx
There's more to onyx than just the black stones you see in jewelry. Onyx marble is known for its many colors and translucent properties. Onyx marble is graded similarly to other types of marble and the grades depend on the quality of the stone.
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Instructions
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Understand the properties of onyx marble. It's the metamorphic form of limestone, which means it has a softer texture. Onyx marble is easily scratched or stained and is considered ornate.
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Know the grading system. Marble is graded into Class A, Class B, Class C and Class D. Big, hard and consistent marbles are in Class A, while small, soft and irregular marbles are in Class D.
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Grade onyx marble. Because of its softness and irregularities, it falls into Class D. These marbles are found in small portions in irregular shapes. They tend to have many faults and require much filling and reinforcement. This is why onyx marble falls into Class D.
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Realize that Class A marbles are not only hard but found in massive quantities. They have minimal faults and require little filling. Marbles found in Class A are consistent looking with few veins.
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Tips & Warnings
Class B and Class C marbles have properties that fall between Class A and D. Putting a marble in the correct class can sometimes be difficult.
If you're thinking about putting onyx marble in your home carefully consider its location. Onyx is porous so it will absorb liquids easily if it isn't properly sealed. It's also calcareous so it will stain easily. For these reasons put onyx in a low-traffic area where it won't be used on a daily basis.
