Birthstone Minerals – Chemical Formula and Appearance 1


Before they are cut and polished into gems, birthstones are natural crystals. Take a look at examples of each of the beautiful birthstone minerals:

January Birthstone – Garnet [Chemical Formula X3Y2(SiO4)3]

Spessartine Garnet Crystal (ButtShark)

Spessartine Garnet Crystal (ButtShark)

Color: any, but birthstone is usually red

Hardness: 6.5 to 7.5

Crystal Habit: Cubic or Rhombic Dodecahedron

The traditional birthstone for January is garnet. This silicate mineral usually is seen as a deep wine or brick red gemstone, but it can occur in other colors, such as orange or green. The rarest garnet color is blue. The colors and crystal form depend on the elements found in the X and Y position in the structure. Common X elements are magnesium, iron, manganese, and calcium. Common Y elements are aluminum, iron, and chromium.


February Birthstone – Amethyst [Chemical Formula SiO2]

Amethyst Mineral Crystal (JJ Harrison)

Amethyst Crystal Mineral (JJ Harrison)

Color: purple

Hardness: 7.0 or lower

Crystal Habit: 6- sided prism

The February birthstone mineral is amethyst, which is a purple or lavender variety of quartz. In nature, the crystal usually gets its purple color from iron impurities. However, quartz can be irradiated to produce the purple color, too. Irradiation is also used to deepen the color of pale crystals.


March Birthstone – Aquamarine [Chemical Formula Be3Al2Si6O18]

Aquamarine Mineral - Beryl Crystal (Rob Lavinsky, iRocks.com)

Aquamarine Mineral – Beryl Crystal (Rob Lavinsky, iRocks.com)

Color: Blue

Hardness: 7.5 to 8.0

Crystal Habit: prismatic or tabular crystals

The March birthstone mineral is the blue gemstone form of beryl, which is called aquamarine. Beryl comes in other colors, such as green (emerald) and pink (morganite). Pale blue aquamarine gets its color from Fe2+ impurities. There is a deeper cyan blue form of the mineral named maxixe, which contains both Fe2+ and Fe3+ ions. Exposure to light or heat can change the ion form and thus the color of the gem. Although the color of the gem fades with exposure to light, it can be renewed if the gem is irradiated.


April Birthstone – Diamond [Chemical Formula: C]

Diamond Crystal (Rob Lavinsky, iRocks.com)

Diamond Crystal (Rob Lavinsky, iRocks.com)

Color: Colorless (ideally)

Hardness: 10

Crystal Habit: Octahedral

Diamond is the name given to the crystal form of the mineral and element carbon. Pure carbon also takes other forms, such as graphite and fullerene. The ideal diamond is colorless, but any color of the mineral may occur, depending on the impurities (usually boron or nitrogen), defects in the crystal lattice, or irradiation. Diamonds also common fluoresce under black light, often glowing with a color different from that of the stone.


May Birthstone – Emerald [Chemical Formula: Be3Al2(SiO3)6]

Colombian Emerald Crystal (Rob Lavinsky, iRocks.com)

Colombian Emerald Crystal (Rob Lavinsky, iRocks.com)

Color: Green

Hardness: 7.5 to 8.0

Crystal Habit: Hexagonal

An emerald is the green gemstone form of the mineral beryl. The green color comes from trace impurities of chromium and sometimes vanadium. Nearly all emeralds contain cracks and fissures, so natural stones are typically filled and coated with oil having the same refractive index as the stone. For this reason, emeralds do not tolerate cleaning well and should be treated with care.


June Birthstone – Alexandrite [Chemical Formula BeAl2O4]

Color Change Alexandrite Chrysoberyl (Rob Lavinsky, iRocks.com)

Color Change Alexandrite Chrysoberyl (Rob Lavinsky, iRocks.com)

Color: Purple or Red to Green Color Change

Hardness: 8.5

Crystal Habit: Orthorhombic

The June birthstone mineral is the rare green gemstone form of chrysoberyl, which is called alexandrite. The color of the mineral changes (pleiochromic) depending on the polarization or spectrum of the light that strikes it. Natural alexandrite changes from red-violet to green. Sometimes orange-yellow is displayed. Most gemstones sold as jewelry do not display the same color change as the natural mineral. Blue to purple is more common in synthetic stones, with little or no green.


July Birthstone – Ruby [Chemical Formula Al2O3:Cr]

Pigeon Blood Ruby Crystal - Corundum (Rob Lavinsky, iRocks.com)

Pigeon Blood Ruby Crystal – Corundum (Rob Lavinsky, iRocks.com)

Color: Pigeon-Blood Red

Hardness: 9.0

Crystal Habit: Trigonal

Only the gemmy, pigeon-blood red form of the mineral corundum is called a ruby. All other colors of gemstones are sapphires. The pink to red color comes from the chromium in the mineral. Rubies almost always contain impurities and also needle-like inclusions of the mineral rutile, which are called “silk”. The alignment of the rutile can produce a catseye or star effect. Heat treatment of rubies used for gemstones is common. Since corundum is relatively easy to produce in a lab, synthetic stones are often used for birthstone jewelry.


August Birthstone – Peridot [Chemical Formula (Mg, Fe)2SiO4]

Egyptian Peridot or Forsterite (Rob Lavinsky, iRocks.com)

Egyptian Peridot or Forsterite (Rob Lavinsky, iRocks.com)

Color: Green (usually lime to olive)

Hardness: 6.5 to 7.0

Crystal Habit: Orthorhombic

Peridot is the gemstone form of the mineral olivine. While most minerals occur in a wide range of colors, olivine is unusual in that it is only ever green. The intensity of the color depends on the amount of iron that is present. Peridot is found in igneous rocks and sometimes in Pallasite meteorites.


September Birthstone – Sapphire [Chemical Formula Al2O3]

Rough Blue Sapphire (Rob Lavinsky, iRocks.com)

Rough Blue Sapphire (Rob Lavinsky, iRocks.com)

Color: Any Except Red

Hardness: 9.0

Crystal Habit: Trigonal

Although the word sapphire comes from a Greek phrase which means “blue stone,” sapphires are any color of gem-quality corundum except red. Colorless sapphire occurs, but metal ion impurities produce a wide array of colors. Common impurities and the colors they produce include iron (blue), titanium (yellow), chromium (purple), copper (orange), or magnesium (green). Although they may be mined, sapphires and rubies can be found by sifting through sediment. This is because the hardness of corundum makes it more resistant to abrasion than most other minerals.


October Birthstone – Opal [Chemical Formula SiO2ยทnH2O]

Rough Ethiopian Opal (Kolekcja - Lech Darski)

Rough Ethiopian Opal (Kolekcja – Lech Darski)

Color: Colorless or Nearly Any Color

Hardness: 5.5 to 6.0

Crystal Habit: Amorphous

Unlike the other birthstones, opal is not the crystal form of a mineral. It consists of hydrated silica and is considered a mineraloid. The silica can be colorless or colored, depending on the nature of impurities. The birthstone is gemstone opal, which diffracts light to produce a play of color.


November Birthstone – Topaz [Chemical Formula Al2SiO4(F,OH)2]

Topaz Crystal Point and Matrix (Rob Lavinsky, iRocks.com)

Topaz Crystal Point and Matrix (Rob Lavinsky, iRocks.com)

Color: Colorless or Any Color (birthstone is blue or amber)

Hardness: 8 (defining Mohs scale mineral)

Crystal Habit: Orthorhombic

Topaz crystals often form prisms and points. The pure form of the mineral is colorless, but impurities can produce nearly any color. Mystic topaz, a rainbow-effect gem, is not a natural color of the mineral, but results from an artificial coating onto a colorless topaz. Topaz is mainly associated with igneous rocks.


December Birthstone – Zircon [Chemical Formula ZrSiO4]

Red Zircon Crystal and Matrix (Rob Lavinsky, iRocks.com)

Red Zircon Crystal and Matrix (Rob Lavinsky, iRocks.com)

Color: blue for the birthstone, but zircon occurs in any color

Hardness: 7.5

Crystal Habit: Tetragonal

As with other gemstone minerals, the color derives from impurities in the crystal matrix. Zircon is a neosilicate, but it almost always contains some amount of the element hafnium. Also, some zircons contain radioactive uranium or thorium. Although zircon is found throughout the Earth’s crust, large zircon crystals are rare.

This birthstone list comes from the American Gem Society. Other birthstones have been used in the past or in other countries.


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One thought on “Birthstone Minerals – Chemical Formula and Appearance

  • Mandy

    Hi Anne, thank you for this interesting and in depth article! Is there any specific criteria that determines when a mineral can be classed as a gemstone? I imagine there are many types of minerals but not all of them are called gemstones. Thanks!

    Under July (Ruby) I think this sentence is missing something – ‘corundum’? “All other colors of gemstones are sapphires.”