Corundum’s primary industrial application is for abrasives. Because corundum hardness ranks at 9 on the Mohs scale, it’s useful for making sandpaper, emery boards, grinding wheels, and other polishing tools.
Most corundum mined is only industrial-quality. Since the first synthetic sapphires of 1902, sapphires have also gained applications in:
Windows
Satellite communication
Scientific tools
Integrated circuits
As an aluminum oxide mineral, the corundum formula is Al2O3. Iron, chromium, vanadium, and titanium are common impurities.
Corundum often forms barrel-like, bi-pyramidal, or unterminated prismatic crystals. Otherwise, it’s granular, massive, or pebble-like.
Some properties — namely refractive indices, birefringence, and density — depend on the source. For instance, different-colored sapphires from Australia can each have different properties.
The highest possible density is 4.06 in reddish-brown sapphires from the Umba River Valley. The lowest possible is 3.80 in Yugoslavian rubies.
Green sapphires tend to have the highest refractive indices, reaching 1.779. The lowest possible value is 1.757 in Sri Lankan blue sapphires.
Here are corundum’s properties:
Mohs hardness: 9
Color: Ruby - Pinkish-red to dark red; Sapphire - Every other color, including colorless, white, gray, blue, blue-green, green, violet, purple, orange, yellow, yellow-green, brown, golden amber, peachy pink, pink, and black
Crystal structure: Hexagonal (trigonal)
Luster: Vitreous (glassy) to adamantine; Vitreous to sub-adamantine (polished); Vitreous (fracture)
Transparency: Transparent to opaque
Refractive index: 1.757-1.779; Lowest = Sri Lankan blue sapphires; Highest = Sri Lankan green sapphires
Density: 3.98-4.10
Cleavage: None
Fracture: Conchoidal or irregular/uneven
Streak: White
Luminescence: Fluorescence & X-ray colors present; Rubies - Varies by source, often red, orange-red, or pink; Sapphires - Sometimes fluorescent and varies by color and source, some red or yellow-orange in X-ray
Pleochroism: Present and very strong in colored stones; Ruby - red/purplish-red to orange-red; Sapphire - Varies by stone color
Birefringence: 0.008-0.009
Optical effects: Asterism - Ruby & sapphire; Color change - Sapphire; Chatoyancy - Ruby & sapphire
Dispersion: 0.018
All red corundum is ruby while all other colors are sapphires.
The ruby subtypes are:
Pigeon Blood Ruby: Vibrant red variety with no undertones
Star Ruby: Ruby displaying asterism, a six-rayed star of light on its surface
Geneva Ruby: The oldest synthetic gemstone, created around 1885
Verneuil Ruby: Synthetic ruby created via flame fusion (a.k.a. Verneuil process)
Flux-Grown Ruby: Synthetic ruby created by dissolving aluminum oxide in a melted flux, followed by ruby crystallization
Unlike ruby, sapphires come in lots of colors. Blue corundum is sapphire, but the colors of sapphires encompass the entire rainbow.
The sapphire subtypes are:
Color-Change Sapphire: Variety that changes color under different lighting (usually blue to violet, but gray- or greenish-blue to brownish-red are best)
Star Sapphire: Variety displaying asterism with 6- or 12-rayed star
Padparadscha Sapphire: One of the rarest types, a pink- to peach-colored variety resembling lotus flower coloring
Kashmir Sapphire: Most valuable sapphires (no longer mined), with vibrant blue and velvety appearance
Ruby symbolizes protection, passion, and manifestation, sometimes called “Stone of Prophecy.” Sapphire symbolizes new love, prosperity, and commitment. Golden sapphire specifically represents wisdom, abundance, and spiritual power.
Ancient Persians believed sapphires came from a pedestal balancing Earth. Kings wore sapphire for protection, while clergymen wore blue sapphires symbolic of heaven.
Biblically, ruby and sapphire appear among the twelve stones in the High Priest’s Breastplate. Sapphire also appears in the twelve “foundation stones” of New Jerusalem.
In multiple Indian religions, the Navaratna (or “nine gems”) is a beneficial, often astrological, talisman containing both yellow sapphire and ruby. Islamic beliefs claim the first man, Adam, created rubies in Sri Lanka after he was banished from the Garden of Eden.
That’s just scratching the surface, as folklore surrounding rubies and sapphires spans dozens of cultures and time periods.
Physically, rubies are popular for treating blood or heart problems. Sapphires are said to treat insomnia and vision issues. Medieval beliefs assigned different healing powers to different fingers to wear sapphire rings on:
Index (Pointer) Finger: Digestive and respiratory systems
Middle Finger: Spleen, liver, cognition
Ring Finger: Circulatory system and kidney
Little Finger (Pinky): Reproductive system and lower body
Experts grade corundum gemstones based on their color, cut, clarity, carat weight, and treatments.
Ruby’s red coloring comes from chromium impurities, while each sapphire color derives from different impurities. The most valuable shade is pigeon’s blood ruby.
Despite being the most common color, blue sapphires still fetch high prices because of high demand. Padparadscha sapphires are next-highest in value. Yellow, green, and white sapphires are more affordable.
Most gem-quality corundum is faceted, which is most valuable. Star ruby or star sapphire must be cut as cabochons to display the “star” properly. Other cuts include beads, carvings, and raw (uncut) specimens.
Corundum has a Type II colored gemstone clarity grade, meaning a few visible inclusions are normal, except in high-quality gems. Many sellers use diamond clarity grades for corundum.
Most inclusions lower corundum’s value, but hematite or rutile inclusions that create asterism or the velvety look of Kashmir sapphires can boost value. Cat’s eye corundum comes about similarly, though these stones actually have imperfect “star” effects.
Common inclusions or imperfections in natural corundum include:
Fingerprints (tiny internal tubes filled with liquid)
Needle-like rutile crossing at 60 degrees
Hexagonal growth lines
Crystals (e.g. zircon, orthoclase, calcite, apatite, pyrite, magnetite, biotite, spinel)
Ruby gems under 1 carat are widely available, but gems weighing 1 carat or more are significantly pricier.
Similarly, sapphire price-per-carat increases by carat weight, with those more than 5 carats being rare.