Diamonds: What’s your type?
By Alethea Inns
Fig 1: The limitations of natural rough, where a cutter must decide to produce three carats of non-ideal-cut diamonds or sacrifice the weight for 2.20 carats of ideal-cut diamonds. Image @ GSI
Human beings love to classify and categorize items—and for good reason! This type of organization helps us better understand common attributes of things and simplify complex data.
Gemstones, of course, are no different. Look at the diamond industry: we have broken down diamonds into 11 grades of clarity, 23 grades of color (not including the fancy color diamond evaluation system, with its nine ranges and countless permutations of hues), and six grades of cut. This adds up to more than 1500 combinations of possible grades for D-Z diamonds alone. These grading attributes, however, do not tell us why a particular diamond is the way it is. For this, we must examine its type.
Unlike one of the worst pick-up lines ever (Are you a keyboard? Because you’re just my type”), diamond type is a science-based classification based on a stone’s atomic makeup. Before you skip this article with the thought of, “No thanks, I’m not interested in learning about inorganic chemistry,” you should realize having sufficient information about diamond types can save you from making a bad off-the-street purchase; help you decide when to send a diamond to a laboratory for additional testing; and even allow you to determine if a diamond is mined, laboratory-grown, or treated.
Indeed, having basic foundational knowledge on this subject can help you become a confident and competent diamond expert. What’s more, you will have plenty of nerdy talking-points for when you find yourself chatting with another industry professional or putting together a sales presentation. Diamond type is fascinating, and determines much about the diamond’s growth, color, and physical properties.
Further, typing is not just for laboratory gemologists. The information gives jewelry retailers and salespeople an edge when communicating with customers, as they are prepared with technical information to help facilitate sales conversations and answer questions presented by educated, informed consumers.
The topic of diamond typing is often covered in an overly technical way, which does not translate to the layperson; however, this is an important subject to better understand diamonds and their properties. So, let’s break it down.
Fig 1: The limitations of natural rough, where a cutter must decide to produce three carats of non-ideal-cut diamonds or sacrifice the weight for 2.20 carats of ideal-cut diamonds. Image @ GSI
What Are Diamond Types?
As most industry professionals already know, diamond is a mineral composed of carbon. The arrangement of carbon atoms within the crystal is symmetrical (Fig. 1). Gem-quality diamonds are typically 99.95 per cent carbon and can be up to 99.99 per cent pure. This makes diamond one of the purest of all gems found in nature. (In fact, diamond is the only gemstone composed of a single element!)
To better understand diamond composition, imagine the carbon atom like a black ball; each ball (carbon atom) is connected to the other carbon atoms in a tetrahedral arrangement (like a pyramid with a triangular base) to form the diamond molecule. As diamond molecules connect, they form a crystal structure (i.e. the crystal lattice), which has the base geometry of a cube. From there, rough diamond crystals are able to take on all sorts of shapes based on the cube depending on the temperatures and pressures at which they form. The most common is the octahedron (Fig. 2).
Beyond carbon, the remaining 0.01 to 0.05 per cent of a diamond’s chemical composition includes other trace elements that are not a part of its essential chemistry. These elements can cause diamonds to be different colors; to fluoresce (nitrogen in specific arrangements); to conduct electricity (boron); or have other unique properties.
These trace elements are sometimes called ‘impurities’—but do not be fooled by the word’s negative connotation. Some of the rarest, most expensive diamonds are considered as such thanks to these ‘impurities.’
A diamond with a perfect crystal lattice made entirely of carbon would be completely colorless. As such, atomic impurities are also sometimes referred to as ‘color centers’ or ‘optical centers’ because they influence the color of the diamond.
The most common atomic impurity in diamond is nitrogen. This is because it is (a) abundant; and (b) easily incorporated into the diamond lattice because it is similar in size and valence shells with carbon (this is a technical way of saying nitrogen fits nicely into the diamond crystal lattice). Approximately 98 per cent of mined diamonds have tens to several hundred parts per million (ppm) of nitrogen.