Crystals and gemstones have been associated with months of the year for almost 2,000 years. Today, we call them birthstones.
Garnet – January’s Birthstone
January’s birthstone is Garnet. Garnets are often found in metamorphic rocks such as schist. Garnets are actually a family of minerals which are all similar. The garnets which are often used as gemstones are typically a dark red color; however, the brilliant green of variety of uvarovite is rare and very prized.
Amethyst – February’s Birthstone
Amethyst is a common, purple form of the mineral quartz. Fairly hard, amethyst is a 7 on the Mohs hardness scale which has a range from 1 (softest) to 10 (hardest). Crystals of the mineral often form in clusters, which are also called druze, and also commonly form in geodes. The purple color of amethyst is due to the presence of ferric iron (Fe3+) in the quartz crystal and can range from light to dark. When heated, amethyst will turn brown, into citrine.
Amethyst is considered a semi-precious gemstone that is most commonly used in jewelry and for collecting. Beautiful samples of amethyst can be found in Brazil, Mexico, Uruguay, Russia, India, Sri Lanka, and the United States.
Aquamarine – March’s Birthstone
Aquamarine is a pale blue form of the mineral beryl. Aquamarine crystals can occur in such rocks as granite and pegmatite.
These beautiful gemstones have a glassy luster (shine) and are either translucent or transparent. These properties make aquamarine a prized stone for all types of jewelry.
Diamond – April’s Birthstone
Did you know that pencil lead and diamonds are made of the same thing? It is hard to believe but they are both made of carbon! The carbon forms in different crystal types or shapes, which is why they are different. Diamond is the hardest mineral, being a 10 on the Mohs hardness scale.
Diamonds form in igneous rocks called Kimberlites and Lamproites. These igneous rocks are typically rich in the mineral olivine and derive from mantle rocks known as peridotites. The peridotite rocks melt deep below the surface (between 90 and 280 miles) of the Earth then rise through cracks in the surrounding rock forming pipe-shaped intrusions. As the magma pushes its way through the cracks, some of the surrounding rocks break off and mix into the liquid magma. Over time, (millions of years) the magma slowly cools into rock with large crystals. Sometimes these crystals include those wonderful gemstones we know as diamonds. This photo shows cores of kimberlite rock from a drilling and exploration project. The rocks cores are stored these long, then boxes for examination. Geologists store the cores in order as they pull them from the ground so that they can view the rocks in one long string and see exactly what is present below the land surface.
The largest diamond ever found is over 7,000 carats which is about the size of your two fists put together. Because of their beauty and strength, diamonds are used for a wide variety of products from jewelry to industrial cutting blades. The photo at the top left shows a group of different sized diamonds which have been cut and faceted.
Emerald – May’s Birthstone
Emerald is a green form of the mineral beryl. Emerald is a hard gemstone that has a glassy luster (shine) and is either translucent or transparent. The elements chromium and vanadium give emerald its green color. Emeralds are one of the most rare and prized gemstones and can be worth more than diamonds if they are pure.
Pearl – June’s Birthstone
Pearls are a beautiful organic gemstone which is formed in a variety of colors and shapes. Pearls form inside of mollusk shells such as oysters and mussels. This unique gemstone is made of primarily the mineral aragonite. Aragonite is the mineral that lines the inside of the mollusk shell. An organic substance, called conchiolin, is also known to line the inside of the shell. When shell linings are made of aragonite and conchiolin together, it is called mother-of-pearl.
To form the gemstone pearl, a grain of sand must get trapped inside the mollusk shell. The aragonite forms in circles around the sand grain. It can take between 2 and 8 years for a large pearl to be formed. Fairly soft, pearls are a 3 on the Mohs hardness scale and have a white streak. The luster, “pearly,” is often used to describe the look of other minerals with similar outward appearance. Pearls can form in both freshwater and salt water and can be round and smooth to oblong and uneven. Common colors of pearls include white, cream and black; however, other colors such as blue, yellow, gray, green, light purple and mauve can also be found.
Ruby – July’s Birthstone
Rubies are one of the hardest known minerals. With a hardness of 9 on the Mohs scale, it is only softer than a diamond. Rubies are known for their beautiful red color. A ruby is a variety of the mineral corundum. The gemstone sapphire is also a variety of corundum and comes in many colors, except for red. Rubies have a glassy luster which makes them shiny. Rubies are most commonly used for jewelry. Rubies are commonly mined in Myanmar, Thailand, Kenya, the United States, Sri Lanka and Tanzania. In 2005, a 440 carat ruby was discovered and thought to be one of the largest gems found to date.
Peridot – August’s Birthstone
Peridot (“pair-a-doe”) is the August birthstone. This lovely green gemstone is a variety of the mineral olivine. Peridot gets its green color from the presence of iron in the crystal’s structure. The amount of iron present determines the intensity of the green color such that the higher the iron content the darker the green color. Interestingly, peridot is only found in the color green whereas most other minerals can be found in more than one color.
Although olivine is common in igneous and metamorphic rocks, the gemstone quality version is much rarer. Arizona has one of the more abundant sources of peridot but the quality is not as high as in other regions, such as Egypt, Myanmar, Burma and Pakistan, which have smaller amounts of the gemstone. The Smithsonian Institution has a gemstone from Egypt that is over 310 carats, which is largest peridot ever found.
Sapphire – September’s Birthstone
Sapphires are a blue form of the mineral corundum. When corundum is red, it is called a ruby. Corundum is one of the hardest known minerals with a hardness of 9 on the Mohs scale. The gemstone sapphire is usually known for its spectacular blue color, but can also come in many other colors, except for red. Sapphires have a glassy luster which makes them shiny and are most commonly used for jewelry. One of the largest sapphires in the world is called the “Logan Sapphire” and is over 422 carats. Many beautiful specimens of sapphire are mined in Myanmar, Madagascar, Kashmire, and Sri Lanka, and Australia.
In the United States, sapphires and rubies can be found in the area of Franklin, North Carolina, which is a popular area for mines.
Opal – October’s Birthstone
Opal is a unique mineral because it forms as a gel in cracks of many different types of rocks. However, opal is most often found in rocks like basalt, rhyolite, sandstone and limonite. High water content is a trademark of the opal. Up to 20% of an opal can be water. Opals come is a wide variety of colors including white, reds, greens, pinks, browns, and blues to name a few. Most opals do not have a specific shape, however, “precious” opal which are used for jewelry do have a round structure to them. Almost all of the world’s opal supply comes from Australia.
Topaz – November’s Birthstone
The November birthstone is topaz. Topaz is a unique gemstone which comes in a huge variety of colors such as orange-yellow, colorless, light blue, pink, brown and green. Topaz has a hardness of 8 on the Mohs Hardness Scale and a glassy luster. Igneous rocks are the most common type in which to find topaz gemstones. Topaz is used for jewelry in its variety of colors.
Turquoise – December’s Birthstone
The December birthstone is Turquoise, a blue to green minerals which is moderately hard and typically shows no specific crystal structure. Turquoise is a 6 on the Mohs Hardness Scale of 1 to 10, has a pale blue to white streak, and a waxy luster. “Cryptocrystalline” is the term often used for minerals like turquoise where the crystals are so small that they cannot be seen. Turquoise can be nicely polished and is therefore, used often in jewelry.
Filed under: Geology / Earth Science, Minerals on August 3rd, 2014 | No Comments »