Gemstones Guide: The Four Precious Stones

Last updated May 6, 2024

Precious stones, also known as precious gemstones and precious gems, are naturally occurring minerals that possess notable beauty, rarity, and durability.

Traditionally, only four gemstones in the world are considered precious: diamonds, sapphires, rubies, and emeralds.

This guide explores the characteristics of each of the four precious gems, and also looks at the key differences and similarities between precious and semi-precious stones.

Why Are Precious Gemstones Precious?

Three engagement rings set with precious stones.
Engagement rings set with the “Big Four” precious gemstones: diamond, sapphire, emerald, and ruby

Fascination with precious stones transcends geographical and historical boundaries. From Ancient Greece and Rome to the empires of Asia and the New World, humans have valued diamonds, sapphires, rubies, and emeralds for thousands of years.

Here are the top reasons why we consider precious stones to be precious:

  • Beauty: Precious stones are aesthetically appealing and captivating to the eye. Their alluring sheen and often vibrant colors distinguish them from more common rocks and minerals.
  • Sparkle: Precious stones are translucent and have the ability to refract and disperse light, which allows them to throw sparkles or flashes of light when cut into faceted gems.
  • Rarity: Precious stones are generally rarer than semi-precious stones (although this is not always the case).
  • Strength: Precious stones are hard and resistant to scratches, chips, and other forms of damage, making them ideal for jewelry use.
  • Symbolism: Precious stones have traditionally been symbols of authority, wealth, and supernatural power, and can be found adorning religious and royal regalia to this day.

What About Semi-Precious Gemstones?

An assortment of natural gemstones including citrine (citrine is yellow), peridot (peridot is green), and tourmaline (tourmaline comes in many colors).
Assorted semi-precious stones

Any gemstone that’s widely accepted as more beautiful than common minerals or rocks, but is not one of the four precious stones, is considered semi-precious. A semi-precious gemstone can be transparent, opaque, or somewhere in between.

Transparent stones are likely to be cut into faceted gems to make the most of their ability to sparkle, while more opaque ones are polished into rounded cabochon gemstones with a smooth, lustrous finish.

In many cases, semi-precious stones don’t possess the beauty, rarity, clarity, color, durability, or value of precious gemstones. However, there are numerous exceptions.

For example, tsavorite garnet. This green color variety of the garnet group is rarer and harder than emerald, with a richer green color, higher clarity, and greater brilliance. And yet, a high quality tsavorite garnet costs only about a tenth as much a high quality emerald.

Another example is paraiba tourmaline, a blue-green clear gemstone that’s about 10,000 times as rare as diamond. Fine quality paraiba tourmaline can sell for up to $87,000 per carat, while high grade white diamonds fetch up to $68,000 per carat.

These examples show that what people consider precious gems (and what we don’t) isn’t solely based on concrete measures like rarity and cost. A lot of why a gem is is classified as precious or semi-precious has to do with tradition, sentiment, and marketing.

The terms “precious gemstones” and “semi-precious gemstones” are still used in the jewelry industry, and are useful descriptions to a point. However, as an informed consumer, you should be aware that a precious gemstone may not necessarily be more beautiful, rare and/or valuable than one that’s considered semi-precious.

Precious Stone #1: Diamond

A white diamond jewelry stone displayed on a dark gray rock.
A round brilliant cut white diamond

Diamonds Overview

Most of the diamonds mined throughout history were formed in the extreme high heat and pressure of the Earth’s mantle. Diamonds are composed of carbon atoms arranged in a covalent lattice structure.

This distinctive structure imparts diamonds with some extraordinary physical properties not seen in other stones. Other minerals, such as graphite and chaoite, are also formed from carbon. But while diamonds are famously hard and translucent, graphite and chaoite are both soft and opaque.

In fact, scoring a perfect 10 out of 10 on the Mohs Hardness Scale, diamonds are the hardest natural substance known to exist.

Diamonds also possess a high refractive index, which allows them to bend light at a greater degree than most other materials. A diamond can also disperse light into spectral colors. These characteristics are what give a high quality diamond cut its signature brilliance and fire.

Diamond is the birthstone for April.

Diamond Colors: Over the Rainbow

Though clear white is the color we naturally associate with a diamond, diamond usually isn’t completely white. The vast majority of mined diamonds have a brownish or yellowish tint, resulting from trace mineral inclusions at the time of their geological formation.

Clear white diamonds are rare gemstones by comparison, and become more expensive the more colorless they are. Diamond color – or more accurately, lack of color – is graded on a scale beginning at D (completely colorless) and ending at Z (noticeably discolored).

Yet white is far from the only valuable color for a diamond. While diamonds that are white with a visible tint of another color aren’t desirable, the game changes when that color reaches a certain saturation.

Then you have what are called fancy colored diamonds, which are rarer and costlier than normal white diamonds. Fancy diamonds come in a rainbow spectrum of color variations: red (the most rare), pink, purple, blue, green, yellow, brown, orange, and black.

Alternatives to Diamond

Diamonds may be a girl’s best friend, but they’re expensive friends to have around. Different gemstones that can be more affordably substituted for diamond include:

  • White spinel, the clear variety of spinel
  • Crystalline quartz, a gem that’s colorless in its natural state
  • White topaz, a hard and transparent white gem
  • Colorless zircon, a white stone with high sparkle
  • White sapphire, the colorless form of corundum

Precious Stone #2: Sapphire

A beautiful sapphire with high clarity.
A step cut blue sapphire

Sapphires Overview

Sapphires are a gem variety of the mineral species corundum. They’re created within igneous and metamorphic rocks, when aluminum and oxygen atoms are subjected to intense heat and pressure to form crystals of aluminum oxide.

When forming, sapphires take a trigonal crystal structure that imparts them with significant hardness and durability. In fact, sapphire is the second-hardest precious gemstone after diamond, ranking at 9 out of 10 on the Mohs scale.

Sapphires have a relatively high refractive index, meaning that they’re good at bending light. However, their light refraction and dispersion is lower than that of diamonds, resulting in less brilliance.

Sapphires also exhibit pleochroism, which means they display color variation when viewed from different angles. This adds depth and complexity to the gemstone’s appearance. Saying that, blue sapphires with strong greenish pleochroism are less desirable, as they make the stone appear less blue.

Sapphire is the birthstone for September.

Sapphire Colors: Blue and Beyond

The default and most common color for sapphires is blue. A blue sapphire gets its color from trace amounts of titanium or iron that were present during the stone’s formation. Other trace elements create different colors.

Blue is the most popular type of sapphire, and is particularly prized if its color is an intense, velvety royal blue. Paler and darker blue gems are not as visually striking or valuable.

Sapphires also come in yellow, pink, green, orange, purple, black, and white (colorless) varieties. Padparadscha is a special type of orange-pink sapphire that’s exceptionally rare. As with blue sapphire, the primary measure of value is the richness of the gemstone’s color.

Most sapphires are heat treated before being sold. Heating a sapphire to high temperatures results in increased color saturation, and can also improve its clarity. Untreated sapphires with high natural color intensity are rarer and more expensive than treated ones.

Alternatives to Sapphire

Other blue gemstones that resemble sapphire but come at a lower cost include:

  • Aquamarine, named after seawater for its blue color
  • Benitoite, a transparent deep blue stone
  • Blue topaz, a clear medium-to-light blue stone
  • Tanzanite, a clear blue or purple gemstone
  • Blue chalcedony, a semitransparent light blue gem

Precious Stone #3: Ruby

A ruby gemstone cut in a marquise shape.
A marquise cut ruby

Rubies Overview

A ruby is actually the same type of gem as a sapphire. A red corundum gem is always called a ruby, whereas corundum gems of any other color are called sapphires. Both form in the same way and have the same chemical composition, hardness, and ability to bend and disperse light.

Another characteristic that rubies and sapphires share is their tendency to contain inclusions, which are small imperfections within the gemstone. These can give rise to special optical properties, such as asterism (the star effect).

Ruby is the birthstone for July.

Ruby Color: Seeing Red

The ideal types of rubies are bright pure red to purplish-red: not too pale, and not too dark. A ruby’s redness is caused by trace amounts of chromium. If iron is included alongside chromium in the gem’s formation, it can cause a less desirable darker color.

While color isn’t considered the most important of the 4Cs for a white diamond, a ruby’s value depends largely on its color. Because rubies are so much rarer than white diamonds, consumers are willing to compromise on aspects like clarity and carat size in favor of better color intensity.

Like sapphires, the vast majority of rubies are heat-treated to improve their color. You should assume that any ruby has been heated, unless shown evidence otherwise.

Alternatives to Ruby

Red semi-precious stones with a similar appearance to ruby include:

  • Garnet, a typically dark red translucent stone (though it also comes in other colors)
  • Red zircon, a clear red gemstone with good brilliance and fire
  • Carnelian, a reddish-brown stone that can be either semi-opaque or translucent
  • Red spinel, a red gem material that resembles ruby (the “Black Prince’s Ruby” that adorns Britain’s Imperial State Crown is actually a spinel)
  • Rubellite, a transparent red gem, also known as red tourmaline

Precious Stone #4: Emerald

A cut and faceted emerald gemstone.
A faceted emerald gemstone

Emeralds Overview

Emeralds are a gem variety of the mineral species beryl, composed of beryllium aluminum cyclosilicate in a hexagonal crystal system, with trace amounts of other elements.

With a moderate refractive index, emeralds can bend light and exhibit a good level of brilliance. However, their light dispersion is on the low side, meaning they don’t display the fiery flashes of light that diamonds are known for.

Emeralds have a relatively high hardness, ranking 7.5 to 8 on the Mohs scale. This makes them quite resistant against scratching.

However, emeralds are more likely to have internal fissures, fractures, and other inclusions than the other precious gemstones. This means they’re more brittle and prone to chipping and breaking.

Emerald is the birthstone for May.

Emerald Color: Going Green

By definition, all types of emeralds are green. Their greenness comes from the presence of trace elements within their crystal structure, primarily chromium and vanadium.

Emeralds can range between yellowish-green to bluish-green, depending on the type and concentration of trace elements. The most desirable emeralds typically exhibit a rich, vivid green color.

While emeralds are not heat-treated, most are oiled to reduce the visibility of inclusions. Oil treatment involves applying a colorless oil to the stone, which helps mask these defects. Oiling can also improve an emerald’s color.

Emeralds may need to be re-oiled every few years to maintain their appearance, as oiling can wear off over time or be affected by heat or chemicals.

Alternatives to Emerald

Semi-precious green gemstones that resemble emerald include:

  • Green jade, an opaque gemstone with a rich green color
  • Prasiolite, amethyst that’s been heat-treated to turn it green
  • Peridot, a transparent yellowish-green gem
  • Demantoid garnet, a type of green garnet
  • Gem silica, a greenish variety of blue chalcedony

Other FAQs About Gemstones

A closeup of assorted precious and semiprecious stones, including amethyst (amethyst is purple) and garnet (garnets come in various colors).
Assorted precious and semiprecious stones

How many gemstones are there?

Because gemstones can be classified in different ways by different people in different places, there’s no universally agreed-upon number for how many gemstones exist. Some sources say there are about 100, while others say about 200, and yet others claim about 300.

What are the most famous gemstones?

Apart from the four precious gemstones, some well-known semi-precious stones include pearl, opal, jade, amethyst, turquoise, amber, rose quartz, and lapis lazuli.

What are the rarest gemstones?

Stones that are especially rare include painite, tanzanite, alexandrite, jadeite, benitoite, red beryl, demantoid garnet, and paraiba tourmaline.

What are organic gemstones?

Organic gemstones are gem materials created with the involvement of organisms or organic matter. For example, amber (amber is fossilized tree resin), jet (jet is fossilized wood), and pearl (pearls are formed inside of marine and freshwater mollusks).

What is the most expensive gemstone?

Fancy colored diamonds are generally the most expensive gemstones. The current record-holder for the highest-priced individual stone is the Pink Star diamond, which sold in 2017 for $71.2 million.

Ready to learn more about gemstones? Check out our guide to the birthstones by month next!