Ruby vs Sapphire Composition

Both ruby and sapphire are composed of the mineral corundum (Al₂O₃), which is made of aluminum and oxygen. The difference in color comes from trace elements present during their formation.

• Ruby gets its red color from chromium. The more chromium present, the deeper the red.
• Sapphires come in multiple colors, depending on trace elements like iron, titanium, vanadium, or magnesium.

Since both ruby and sapphire are corundum, what determines if a stone is classified as a ruby or a sapphire?

• If the corundum is red, it is called a ruby.
• If the corundum is any color other than red, it is called a sapphire.
• Pink corundum is sometimes debated between being a ruby or a pink sapphire, depending on the gemological laboratory grading it.

This means that all rubies are corundum, but not all corundum are rubies—they could be classified as sapphires instead.

The main difference in composition between ruby and sapphire is the presence of chromium in rubies.

• Rubies contain chromium (Cr), which gives them their red color and fluorescence under UV light.
• Sapphires lack high chromium content, instead containing iron and titanium, which produce blue hues, or other elements creating different sapphire colors.

Higher chromium content in rubies results in:

• More intense red coloration (such as pigeon blood ruby).
• Slight fluorescence, making rubies appear even more vibrant in sunlight.
• Slightly lower hardness than sapphire due to chromium’s effect on the crystal structure.

Sapphires, especially blue ones, get their color from iron and titanium impurities, which do not cause fluorescence.

Both ruby and sapphire share the same trigonal crystal structure and rank 9 on the Mohs hardness scale, making them extremely durable gemstones.

Since they share the same atomic structure, both rubies and sapphires are highly resistant to scratching and wear, making them ideal for jewelry.

Although both gemstones are corundum, they form under slightly different geological conditions, influencing their rarity and distribution.

• Rubies form in metamorphic and igneous environments rich in chromium but low in iron.
• Sapphires form in a variety of geological settings, including basaltic deposits with higher iron content.

This difference explains why rubies are rarer than sapphires, as the specific conditions needed for high-chromium, low-iron corundum to form are less common.

The rarity of high-quality rubies makes them more expensive than most sapphires, with only Padparadscha and Kashmir sapphires reaching similar price levels.

Since rubies are only red, their color range is limited, whereas sapphires come in a wide variety of colors.

Additionally, treatments such as heat treatment are common for both rubies and sapphires to enhance color and clarity. However, rubies may also undergo lead-glass filling to improve transparency, a treatment not commonly seen in sapphires.

Ruby and sapphire share the same chemical composition as corundum, but their color differences arise from the presence of chromium in rubies and iron/titanium in sapphires. While ruby is always red, sapphires can appear in a range of colors, making them one of the most diverse gemstone families.

• Rubies contain chromium, which gives them their rich red hue and fluorescence.
• Sapphires lack chromium but can contain iron, titanium, and other trace elements, leading to various colors.
• Both gemstones rank 9 on the Mohs scale, making them highly durable.
• Rubies are rarer than sapphires, contributing to their higher price in the market.