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Cleaning Rough Diamonds

Cleaning Rough Diamonds

I have talked in the past about how the diamond industry employs an estimated 10 million people around the world, both directly and indirectly. Indirectly, there are countless people and companies who earn their incomes from providing services in support of the diamond industry and the companies that work within it. Many of these supporting services are instrumental in bringing diamonds from mine to market, yet some people may not be aware of the many helping hands that diamonds go through on their journey to a retail shop, and the many companies that provide supporting services to that end. In my next series of articles, I want to take a look at some of the companies that provide indirect support to the diamond industry, to help rough producers and polished manufacturers maximize the value of their product.

Consumers are of course familiar with polished diamonds in their most brilliant form. However, in the earliest stages of a diamond’s journey, it comes out of the ground with a far less dazzling appearance, and must first go through an intense cleaning process to let rough graders see the true merits of each stone. Mined rough diamonds are often quite dirty, covered in the residue of the kimberlite home they lived in for billions of years before being unearthed, as well as in the organic material and oxidation absorbed during their growth. The process of cleaning diamonds is a mixture of art and science, often done by third party companies in diamond centers around the world, as well as at the mines themselves. Let’s take a look at a few of the processes being used today to clean rough, and get it ready for sorting and valuation.

The challenge with cleaning rough diamonds is not just to clean the surface of the stone, but to actually clean within the cracks of the diamond. During both the growth of the stone, and its journey to the surface in a violent kimberlite explosion, the stone can absorb many different materials within the cracks and crevices that penetrate the middle of the diamond. This makes it very difficult to accurately see the internal quality of the stone. Properly cleaning the diamonds adds tremendous value to a rough production, likely worth more than $1 billion annually compared to selling rough as it comes from the earth.

In order to clean these impurities from within the rough diamond, the stone must be subjected to vaporized gases, which can get inside these microscopic cracks to do their work. The long-standing method of achieving this gas vaporization is a multi-stage process. There are many different variations of this process, and each cleaning company will typically maintain their own unique ‘recipe’ of chemicals and processes. At the core of the process is the mixture of chemicals in an autoclave. These chemicals usually consist of mixed ratios of hydrogen peroxide, an alkaline reagent, and a mixture of concentrated sulphuric, hydrochloric, and nitric acids. These chemicals are boiled at 250-500°C. This boiling vaporizes the acids, which expand and build up pressure within the sealed autoclave. This pressure forces the gases into the cracks of the stones, where they digest the organic materials within.

A second stage rinses the diamonds with distilled water before another round of boiling, often with a mixture of nitric and hydrofluoric acids. Hydrofluoric acid has been shown to be very effective in cleaning diamonds, but is also a very noxious and dangerous chemical, as is therefore not supported or used by all diamond producing companies due to concerns for worker safety. The whole process is somewhat time consuming, and can take 12 to 24 hours to fully complete.

More recently, companies have begun working with new cleaning technology using microwave radiation. The chemicals used with this process are largely the same as the traditional boiling method. However, instead of subjecting the autoclave to high heat, it is instead subjected to microwave radiation. A 1,200-watt microwave can generate strong high-frequency radiation at the centimeter and sub-centimeter wavelength. This has been shown through many other industrial applications to accelerate the course of many chemical reactions, with an increased thermal exponential factor, and the influence of harmonic oscillations caused by the radiation. This is achieved with much lower heat than required for boiling, often not exceeding 210°C. More importantly, the microwave method can achieve results in approximately 40 minutes, versus up to an entire day using the traditional method. This new microwave method is slowly becoming the standard in the industry as more and more companies adopt this time and money saving alternative.

Boiling facilities exist in all the major diamond centers of the world. In Antwerp, for example, there are as many as four separate companies offering cleaning services to both rough and polished dealers and producers. Most, but not all, mining companies also employ some variation of cleaning and/or deep boiling at the mine site. It is not uncommon for rough diamonds to go through multiple rounds of cleaning before being presented to the final buyers, as the sorting, sizing, and valuation process can make them dirty, and a final gentle cleaning process adds value at the final stage before sale. Most boiling companies also offer additional services such as laser engraving and digital photography.

 The views expressed here are solely those of the author in his private capacity. No one should act upon any opinion or information in this website without consulting a professional qualified adviser. 

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