A Guide to Precious Stones and How to Choose Diamonds, Emeralds, Sapphires, or Rubies

The world of jewellery, precious metals, and gemstones can be quite overwhelming. Although the advice from a famous U.S. investor “if you don’t know jewellery, know your jeweller” still holds true, having a basic grasp of precious stones can be very important. This guide includes a basic classification of gemstones, describes the big four - diamonds, rubies, sapphires, and emeralds - and tells you what to consider when thinking about a purchase. Learn about the differences between natural and synthetic gemstones, and about gemstone certification.

Drahokam v třpitivých barvách

Basic classification of precious stones

Diamonds, rubies, sapphires, emeralds

Today, gemology and jewellery divide gemstones into two main categories - diamonds, rubies, sapphires, emeralds (also known collectively as the “big four”), and other precious stones, despite the fact that certain stones which fall under the “other” category can be significantly pricier than what’s recognised as a gemstone. Diamonds are the only ones that have a single unified evaluation system and sometimes are set apart as a category on their own, delegating rubies, sapphires, and emeralds to be the “big three”.

Other stones - semi-precious stones

Other stones (not in the special gemstones category - the big four) are known in the business under the unofficial name semi-precious stones. The category includes over 100 stones - e.g. the agate, alabaster, amber, amethyst, crystal, garnet, jadeite, jasper, lazurite, malachite, moldavite, nephrite, opal, rose quartz, or tourmaline. Organic stones, such as amber and pearls, also tend to be termed semi-precious stones.

Diamonds, rubies, sapphires, and emeralds - the big four

Sapphires, diamonds, rubies, and emeralds

Historically speaking, sapphires, diamonds, rubies, and emeralds have been recognised as the most valuable gemstones. These four stones are translucent and judged primarily by the richness of its colour (the richer the colour, the more expensive the stone). Diamonds are an exception—while colourful diamonds are highly prized, most of the stones used in jewellery-making are rated by their lack of colour (the less colourful the stone, the more expensive). 

Sapphires, diamonds, rubies, and emeralds are evaluated by three primary and three secondary criteria. These criteria are what determines the stone’s price and they are very important.

The primary criteria in evaluating sapphires, diamonds, rubies, and emeralds


In terms of aesthetics and beauty, the big four are assessed by:

  • Clarity: The stone’s clarity is determined by the amount and size of foreign bodies (the so-called inclusions) in is structure
  • Colour: The colour intensity, where the colour is neither too light, nor too dark (doesn’t apply to colourless diamonds)
  • Cut: The stone is assessed by its appearance in reflected light, its quality determined primarily by the quality of the cut (the better the cut, the better the stone’s ability to reflect light).

Rarity and availability in the market

  • Generally speaking, the rarer and less available a gemstone is in the market, the higher its price. 

Hardness and resistance 

  • The stone is evaluated for its hardness and resistance against chemical substances.

Secondary evaluation criteria

The secondary criteria (applicable primarily to colourful gemstones) for evaluating sapphires, diamonds, rubies, and emeralds are as follows:

Weight (carat)

In gemology, a single carat is equal to 0.20 grams. The bigger the weight, the more precious the stone. However, heavier stones aren’t necessarily higher quality as that is judged by other criteria.

Gemstone treatment

Gemstone treatment is a term for any process, other than cutting and polishing, that improves the stone’s appearance, colour, or clarity or changes its visual aspect or refines the stone (colour or clarity), durability, and value.

Today, the overwhelming majority of gemstones are artificially treated for a better appearance. There are many techniques to achieve this (heat treatment, radiation, colouring, oiling, resin application, etc.). Recognising that such treatment was applied is difficult and the information should always be provided to the customer before the purchase.


The gemstone’s origin and from which region or quarry it comes can also affect the stone’s price.

Sapphires, diamonds, rubies, and emeralds - basic description

Diamant v pinzetě klenotníka


The price of diamonds is determined by the so-called 4C:

  • Colour 
  • Clarity 
  • Cut 
  • Carat

Traditionally, white (clear) diamonds are valued the most. The general rule goes that the less colourful a diamond is, the clearer and more valuable it is. Until recently, colourful diamonds weren’t considered valuable at all. They’re more popular today and sell for high prices.

Diamonds are the brightest of all translucent gemstones and the hardest material on Earth. They’re rated 10 at the Mohs scale of hardness. The only thing that can cut a diamond is another diamond.

The name is derived from the ancient Greek word "adámas" (meaning “indestructible”).

Diamonds consist of pure carbon and were formed more than one billion years ago, 100 miles under the surface of the earth. Over the years, volcanic eruptions brought them to the surface. Interestingly, graphite also consists of pure carbon though its atomic structure is entirely different and the mineral occupies the opposite end of the hardness scale.

South Africa, Botswana, and Russia are the main producers of diamonds. The stone is also mined in India, Brazil, China, Thailand, or North America.

Prsten se safírem


A type of corundum, sapphires are known primarily for their blue colour that is the result of a titanium and iron content. But they actually come in all colours (though red corundums are called rubies). In fact, you can encounter green, yellow, purple, orange, pink, or even grey and black sapphires.

Some sapphires change colour, typically going from blue to purple when moved from daylight or fluorescent light to the light of the bulb. 

Sapphires are the second hardest mineral on Earth (rated 9 at the Mohs hardness scale). 

The name is derived from the Greek word "sappheiros" (meaning blue stone).  
Many quality, highly prized sapphires are mined in Sri Lanka. Other significant mining sites can be found in Australia, Thailand, Cambodia, Myanmar, etc.

The most famous sapphire ever mined is likely the 536ct Star of India, found in Sri Lanka, or the 182ct Star of Bombay.

There is no standardised system of sapphire classification, and so it’s important to know what to focus on when thinking about purchasing sapphires or sapphire jewellery.

Basic parameters in evaluating sapphires


Like diamonds, sapphires are cut to be the same shape. There’s no standardisation governing the classification of sapphire cuts as each stone is cut in a way that maximises its colour. 


A sapphire’s colour is the most important factor determining the stone’s quality. Velvet- and purple-blue are valued the most. Most people are familiar with the blue variety, but in fact sapphires come in all colours. Red is the only colour that doesn’t apply as red corundums are classified as rubies


Just like rubies, so are most sapphires today treated with heat. Unheated, deep blue sapphires are sold for enormous sums. 


Unlike diamonds whose clarity is rated under 10x magnification, sapphires are judged by their visual appearance. Sapphires typically do have inclusions but high clarity still ensures high price.

Carat weight

Sapphires tend to be heavier than diamonds (hence why a 3ct diamond seems larger than a 3ct sapphire). Unlike other properties, the weight can be measured precisely. Most blue sapphires weigh less than 5 carats.

Zelený smaragd


Emeralds (a variety of the mineral beryl) get their impressive green colour from chromium. In the 1st century AD in Naturalis historia, Pliny the Elder wrote on the subject of emeralds that “nothing greens greener”.

The richer the colour, the more valuable the emerald. Some emeralds are almost colourless or come in shades of yellow and blue. Emeralds are rated between 7.5 to 8 at the Mohs hardness scale.

The name is derived from the Greek word "smaragdos" (meaning green gemstone). 

The most famous emerald jewellery includes the Spanish Inquisition Necklace, the Hooker Emerald Brooch, or the The Crown Of The Andes.

Most emeralds today are mined in South America. Columbia, especially, is considered the most valuable source. Additionally, Columbian emeralds are the standard according to which all other emeralds are judged.

As is the case with sapphires, there’s no standardised classification system so it’s important to know what to focus on when shopping for emeralds.

Basic parameters in evaluating emeralds


As is the case with rubies, the colour is the most important parameter. It should be distributed evenly and not too dark. It can be anything from light to rich green. The deeper and greener the colour, the more valuable the emerald.


The right cut maximises the stone’s desirable green colour. The so-called emerald cut is especially popular as it brightens the stone while minimising any inclusions.

Carat weight

The price difference between small and large emeralds is significant. Some of the best-known emeralds in private collections or museums weigh hundreds of carats and are considered invaluable.


Typically, clarity isn’t assessed under 10x magnification as is the case with diamonds. Instead, emeralds are evaluated by eye only. Inclusions are so common that they don’t affect the price much.

Červený rubín


Rubies are the most sought-after gemstones after diamonds, with large rubies being even more precious than diamonds. The price per carat is usually the highest out of all colourful gemstones. Rubies are a red variety of corundum - essentially sapphires though they’re classified separately due to their beautiful colour.

In its purest form, the mineral corundum is colourless. It’s the trace element of chromium which gives rubies their red colour.

Rubies are rated 9 at the Mohs hardness scale.

The name is derived from the Latin word "rubeus" (meaning red). In Sanskrit, rubies are called "ratnaraj" (the “king of precious stones”). 

Rubies are mined in Thailand, Vietnam, Brazil, Australia, India, or Africa. The best-known rubies, often formed in marble, come from Myanmar in the Himalayas and from northern Vietnam.

Prized rubies also decorate the crown and sceptre of the Bohemian crown jewels.

As is the case with sapphires and emeralds, there’s no standardised system of ruby classification. So what should you focus on when shopping for rubies? 

Basic parameters in evaluating rubies


Colour is the most important factor in determining a ruby’s value. The highest-quality rubies are clear, bright to light purplish red and boast a vibrant richness of colour. A quality ruby’s colour can be neither too dark, nor too light. If the colour is too dark, it affects the stone’s brightness. On the other hand, if the colour is too light, the stone is considered a pink sapphire. Burmese ruby is often seen as the brightest, most valuable shade.

Carat weight

The bigger the carat weight, the more expensive the stone. Quality rubies weighing over one carat are very rare, and so the price rises significantly the heavier a stone is.


Rubies usually come in the mixed cut, with a brilliant crown and step-cut pavilions.


Clarity is an important factor in assessing a ruby’s value which is impacted by small imperfections (inclusions) in the stone. The value depends on the visibility of inclusions. If inclusions affect clarity or brightness, the gemstone’s value drops significantly.

Flawless rubies are extremely rare since almost all rubies contain imperfections. This should be kept in mind when shopping for rubies, to make sure you aren’t accidentally purchasing a synthetic gemstone.

Natural and synthetic gemstones

Drahokam safír

Synthetic (lab-grown) gemstones 

Synthetic gemstones are materials grown by humans in a lab. The chemical, optical, and physical properties are essentially the same as those of the natural mineral counterpart, although in some cases there may be other compounds present as well.

Synthetic gemstone crystals have been produced since the late 19th century, often responding to the needs of industries other than jewellery. In 2015, the sales of lab-grown diamonds amounted to less than 1% of the entire global sale of jewellery diamonds; today, the percentage is almost 20%.

The slightly different properties (such as the lack of natural inclusions) allow gemologists to distinguish synthetic gemstones from natural ones. However, laymen can’t tell the difference with the naked eye.

At any rate, when shopping for diamonds or any other gemstones you’re entitled to know whether you’re purchasing the natural or lab-grown variety, especially since the difference affects the price significantly.

Apart from synthetic gemstones, jewellers also use imitations to mimic real gemstones with synthetic or different natural materials.

Basic differences between natural and synthetic diamonds


Natural gemstones formed naturally over thousands of years, deep underground. In this respect, they’re incredibly unique and their price reflects that. Synthetic (lab grown) gemstones are grown by humans in laboratories in ways that make sure the result closely reflects the natural counterpart. The latter has the same chemical and physical properties as natural diamonds but forms much faster and the price is much lower. 


Lab-grown gemstones are almost perfectly clear and are nearly flawless. This is a plus if perfection is what you seek (as opposed to nature’s perfect imperfections).


Lab-grown diamonds are much more affordable - they cost approximately 60% to 85% less than natural gemstones of the same carat weight and quality.

Diamond and other gemstone certification

Regardless of whether or not you view yourself as knowledgeable about gemstones, securing a certificate whenever purchasing a pricier diamond or any other gemstone is a must.

Prsten s diamantem na certifikátu diamantu

Certificate from a respected laboratory

Certificates confirm a gemstone’s origin and quality and are vital in negotiating insurance or subsequently selling the stone. They give you a reason to trust in the purchase’s basic transparency and the facts being certified. Of course, they don’t guarantee that the stone in question is worth the price you paid or intend to pay for it, and neither do they guarantee you’ll like the stone in a few years’ time.

A gemstone certificate must be issued by an independent licenced gemology lab. Some certificates only confirm the stone’s authenticity and weight. Others (especially those issued for diamonds) also specify colour and clarity grades or the origin.

Diamond and other gemstone evaluation and certification is always subjective (there’s no definition that mathematically determines, for example, what a specific colour or brightness are). Still, gemstone evaluation is relatively fixed and there shouldn’t be any marked differences between different certificates issued for the same gemstone.

You can also request a lab certificate at any time after completing a purchase. There are many companies issuing gemstone certificates, and the evaluation outcomes are often quite different.

You should always purchase gemstones with certificates issued by a respected lab. The highest regarded laboratories include the GIA (Gemological Institute of America), IGI (International Gemological Institute), or GIE (Gemological Institute of Europe).