How to Buy a Diamond – The Gem Guide

As a qualified Diamond grader and tutor, I have had the privilege of looking at hundreds of Diamonds and I wanted to created the most complete guide on How to Buy a Diamond.

Diamond Facts

It is always useful to understand a bit of information about a gem before you go and buy one, so here is a little bit of information about Diamonds.

  • Diamonds are the hardest known natural material in the world, they measure 10 on the Mohs scale and are considerably harder than Ruby and Sapphire. Even though diamonds are very hard they can still be chipped but only a diamond can scratch another diamond.
  • Diamonds are not only very hard, they are also very durable which means that they are perfect for any type of jewellery, including engagement rings.
  • Colour: Colourless, can have varying degrees of yellow or brown tint. Fancy coloured diamonds include red, yellow, pink, blue and green.
  • Birthstone Month: April
  • Anniversary: 60th and 75th
Round Brilliant Cut Diamond. Credit: GIA

Understanding the 4 C's

When you are looking to buy a diamond, one of the things you will undoubtedly come across is the 4 C's, but what are they and what do they mean?

The 4 C's - Carat

Carat is a unit of measurement to weigh gemstones and 1 carat is equal to 0.2 grams. For stones weighing less than 1 carat, they are referred to as points and this isn't done to confuse you, it is just an easier way to describe these smaller stones and is easy to understand. 1 carat is divided into 100 points, this means that a 25 point diamond weights 0.25 or a quarter of a carat and a 50 point stone weight 0.5 or half a carat.

It should also be noted that the price per carat of diamonds do jump when the weights go over certain thresholds such as 1 carat, 2 carat and 5 carats. There are also smaller jumps at smaller thresholds such as 0.5 carat and 1.5 carats for example. So a 0.98 carat diamond cost quite a bit less than a 1.01 carat diamond, even though visually you won't see any difference.

Also be sure to know the difference between total carat weight, often displayed as TCW and the weight of the main stone. If the stone is set with lots of small diamonds, they may advertise the TCW as 1 carat but the main stone may weigh less than half a carat.

The 4 C's - Cut

The often overlooked C but cut is a very important part of assessing the quality of a diamond. There are two parts to cut, the first is the style in which the diamond has been cut and the quality of the cut and finish of the diamond.

Style of Cut

There are many different styles of cut you can choose for your diamond and most people have a style that they like more than others.

The Round Brilliant Cut Diamond

Round Brilliant Cut Diamond

The round brilliant is the most popular cut style for diamonds and the popularity is not really influenced by fashion as can be found with some other styles. There is also no other cut of diamond that has had as much research and development to try and product the perfect cut diamond. Due to the way the round brilliant is cut, it returns the most amount of light to the person looking at the stone and that is why they have so much fire and brilliance. When cut perfectly, a round brilliant will have a total of 57 facets, 33 on the crown (the top) and 24 on the pavilion (the bottom) of the diamond.

The Round Brilliant is the benchmark when it comes to optimising the brilliance of a diamond. The techniques used to perfect the round brilliant have been applied to other shapes of diamond and the following 8 styles are know as modified brilliant cuts.

The Princess Cut

Princess Cut Diamond

The second most popular choice of diamond cut is the princess cut. The popular square shape can be traced back to the early 1970's but the final princess design was completed in 1979 and it is a modified version of round brilliant cut. Unlike the round brilliant, there can be some variations in the princess cut as the facet layout and design can vary between diamond cutters. Care must be taken with a princess cut as the points of the diamond can be quite fragile, so the right type of setting that protects the corners is a must.

The Cushion Cut

Cushion Cut

The cushion cut was a style of cut that was liked by a few people but was not an overly popular choice, until November 2017. What happened in November 2017, the 27th of November to be exact? On this day it was announced that Prince Harry was engaged to Meghan Markle and the beautiful engagement ring had three cushion cut diamonds set in 18ct yellow gold. In the time since, the popularity of cushion cuts went through the roof. The modern cushion cut is a modification of the round brilliant cut.

Pear Cut

Pear Cut

Oval Cut

Oval Cut

Radiant Cut

Radiant Cut

Trillion Cut

Trillion Cut

Marquise Cut

Marquise Cut

Heart Cut

Heart Cut

Step Cuts

Step cuts are another style of cutting diamonds and they get the name from the way that the pavilion facets are cut. Step cuts do not have the kind of fire and brilliance that is seen in brilliant cut diamonds but they do have a large window into the stone, this means that inclusions are far easier to see so it is generally only stones with very good clarity that are cut in these styles.

Emerald Cut

Emerald Cut

Asscher Cut

Asscher Cut

Baguette Cut

Baguette Cut

The brilliant and step cuts are the most popular styles of modern cuts. If you want a real vintage look to your ring, then an Old European or Mine cut might be perfect for you.

Quality of Cut

The other part of cut is the quality of the cut, which is rated as Poor, Fair, Good, Very Good and Excellent. There are three sub-categories to cut quality and they are:

  • Proportions - this is the dimensions of the stone and a diamond with poor dimensions can look pretty bad. You ideally want the proportions to be either Very Good or Excellent, especially if you are buying a round brilliant as there are ideal proportions for this cut.
  • Symmetry - this is how symmetrical the diamond is and you ideally you want a Very Good or Excellent grade.
  • Polish - this is how good the final polish is on the diamond. If the final polish is not done to a good quality, there will be polishing marks on the stone which do not look very good. You ideally would want a diamond with a Good, Very Good or Excellent polish grade.

Anatomy of a Diamond Cut

You may be directed towards buying a triple excellent cut diamond but honestly it would take an experienced diamond grader to tell the difference between a very good and excellent cut diamond. It is best to avoid poor or fair graded stones as the cutting defects will be quite noticeable.

The 4 C's - Colour

When we talk about the colour in white diamonds, we are talking about how free from yellow or brown colour the diamond is. There are two standard colour grading charts but it is the GIA chart that is most commonly used and the colours range from D (colourless) to Z (strong yellow or brown tint), the choice of colour you want is completely down to personal preference but if you want a colourless diamond then you want a D,E,F,G or H colour as these are all colourless when viewed through the top of the diamond. It is only from I colour onwards that you will be able to see a hint of yellow or brown.

If the stone has a hint of any other colour such as blue, pink or green then a different colour grading system is used.

GIA Colour Grading

The diagram above is how GIA grade fancy colours and they look at a combination of the following:

  • Hue - which is the general appearance of the colour, for example blue or pink. There may be more than hue present and you may seen greensih blue on a report, the last colour mentioned is the strongest body colour seen in the diamond and the preceding colours are weaker hues that are visible.
  • Tone - describes how light or dark the colour is.
  • Saturation - describes how strong the colour seen is, from faint to vivid.

Ideally when buying a coloured diamond, you want the colour to be either fancy, fancy intense or fancy vivid as these are when the stone possesses a strong, beautiful colour that is not too light or dark.

The 4 C's - Clarity

Clarity is how free from inclusions and blemishes the diamond is. Before you as, an inclusion is an impurity and/or internal breakage in a diamond and a blemish is if the are any imperfections on the surface of the diamond. As with colour there are two main grading systems used but the GIA one is the most popular and looks like this:

  • Flawless (F) - diamond is free from any inclusions or blemishes
  • Internally Flawless (IF) - diamond is free from any inclusions but has a very small surface blemish
  • Very Very Slightly Included (VVS) - There are very very small inclusions within the stone that is very difficult to spot with a 10x loupe
  • Very Slightly Included (VS) - Very small inclusions that are difficult to spot with a 10x loupe
  • Small Included (SI) - The diamond has small inclusions that are easy to see with a 10x loupe
  • Imperfect/Included (I) - The diamond has a easily noticeable inclusions, sometime visible by the naked eye, stone also has beauty and/or durability issues

You may also see a 1 or 2 next to VVS, VS and SI on some diamonds. These are subcategories within the grading criteria and the number and location of inclusions can determine whether it is graded as a 1 or 2. Imperfect diamonds are separated into three subcategories, I1, I2 and I3, the higher the number the more the beauty and durability of the diamond is affected.

I highly recommend that you avoid an I1, I2 or I3 diamond as you may face some issues with the diamond in the future. My general advice is to go for an SI1 minimum as you are less likely to have issues with diamonds with higher clarity grades.

What is the most important of the 4 C's?

I have seen a lot of people saying that one of the 4 c's is the most important or you should prioritise one over another. Honestly, none of them are the most important. My advice to clients is always to get the best compromise between the 4 for your budget as I feel that this way you get the best quality stone for your money.

A lot also comes down to personal preference, especially when it comes to colour as some people prefer a tint in their stone where as some prefer it to be as bright and white as possible. I say this on pretty much all my guide, when it comes to buying your perfect Diamond, buy what YOU like! Not what the sales person says is the best or what other people like, you will be the one wearing it so make sure you like it before spending your hard earned money.

Beyond the 4 C's

Even though we use the 4 C's as the primary criteria for assessing the quality of a Diamond, there are more elements to consider when buying a diamond and I will cover all of these below.

Lab Reports

The majority of diamonds for sale in the UK, especially stones larger than half a carat are usually accompanied by a lab report. When looking at the lab report, it is always worth checking the reputation of the laboratory as some are not as good as others. Reputable gem labs include GIA, HRD, IGI, SSEF, Gübelin and Anchorcert. Before buying the diamond, ask for the report number so that you can check it with the gem lab to make sure the diamond matches the report.

GIA Lab Report

Many retailers will refer to lab reports as certificates or certified diamonds which is an incorrect description. If the were certificates, then they would be a statement of fact. However, they are the opinion of an experienced diamond grader/s who have viewed the stone and it should be used as a guide.

Don't buy a Diamond just from the report

Diamond reports are a useful tool but buying one just from the information on the report is not the best way to buy one. It is always best to view the stone in person, preferably with the guidance of someone who knows what they are looking at. The reason I say this is because I have seen diamonds that look great on the report but when you actually look at them, they are not that nice or they look dull or lifeless.

Fluorescence

I see a lot of information on the internet that having fluorescence in a diamond is a bad thing and that it can affect the colour of the diamond. The only time it impacts the colour of the diamond is if it is very strong blue fluorescence as this can reduce the yellow tint in some diamonds. If the stone has faint to medium fluorescence, this will not impact the colour of the stone but you may find that a diamond with medium fluorescence will be cheaper than the equivalent quality diamond with no fluorescence.

Treatments

Although not a common sight in the UK marketplace, treated diamonds can be found in jewellers. It is recommendation that you should avoid buying a treated diamonds as they only use treatments on low quality diamonds, even though they may appear like good value it is always better to buy a good quality untreated diamond. Every retailer or jeweller should disclose if the diamonds you are looking at are treated. If the diamond has a lab report then it will state whether or not the stone has been treated.

Clarity Treatments

  • Fracture filling, this is where they fill surface reaching fractures with a high lead content glass. While it does reduce the appearance of the fracture, it does not improve the clarity of the diamond. Fracture filling is not a permanent treatment and the glass can be removed from the stone and all reputable gem labs will not give a fracture filled stone a clarity grade.
  • Laser drilling, this is where they drill a laser down to a dark inclusion within the stone and either burn away or bleach the inclusion to make it less visible. As with fracture filling, while the stone may look better the clarity grade does not improve and this treatment actually adds another inclusion in the form of the laser drill channel. Unlike the fracture filling, laser drilling is a permanent treatment and can be clarity graded by gem labs.
  • On rare occasions, diamonds have been both laser drilled and then fracture filled.

Colour Treatments

  • Coating, a very old type of treatment where the girdle and sometimes the pavilion of the diamond is coated in a blue paint type material. This is done to stones with a strong yellow tint and blue counteracts yellow and gives the stone a whiter appearance. Coating can wear off over time and will eventually reveal the true colour of the diamond.
  • High Pressure High Temperature (HPHT), this kind of treatment is often done to diamonds that have a brown colouration and the stone is subjected to temperatures and pressures similar to that under which they form and the result of this is that the brown colour is either reduced or completely removed from the diamond. The resulting colour is stable.
  • Irradiation and Annealing, these treatments are done to create fancy colour diamonds, so if you are planning on choosing a pink, blue or orange then always check the diamond report to see if the colour is natural. In this process the diamond is first exposed to some form of radiation which alters the colour, commonly to green but also bluish greens and black. If another colour is desired then the stone may subsequently be heated to between 500°C and 1200°C to produce yellows, oranges, browns and pinks.

Simulants

Due to the value and rarity of diamonds, many other gemstones both natural and manmade have been used to simulate a diamond. Glass was a common simulant for diamonds for hundreds of years but it has been replaced by more convincing gemstones. The two most common simulants on the market today are Cubic Zirconia (CZ) and Synthetic Moissanite, both of which are manmade. There are quick and easy to use tools such as thermal and electrical probes that can be used to test if the stone you are looking at is a diamond or not. Other gemstones such as colourless Sapphire, Spinel and Quartz, basically any gemstone that is colourless can be used to imitate diamond.

Synthetic Diamonds

These are manmade diamonds and unlike simulants are actually a diamond. The difference is that they have been made in a lab rather than mined out of the earth and as the processes have been refined, they are producing some very good quality stones. Whether or not you should buy one is down to you and if you are buying one for ethical or environmental reason, I strongly advise you do some in depth research into this subject and don't just read the marketing material produced by the companies producing these diamonds, so that you can come to an informed decision.

Other posts from The Gem Guide series:

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