How are Gemstones Formed?

Gemstones are specific chemical formulations of minerals. These minerals form in the mostly in the Earth’s crust with the exception of 2 which form in the mantle: diamond and peridot. The mantle (~80% of the Earth’s mass) consists mainly of molten rock (magma) and has a solid outer layer. Regardless of formation location, all gemstones are mined in the crust.

The Earth's layers

Gems Formed in the Earth’s Crust

How are gems formed in the Earth’s crust?

Water.

Rain is an important component in the Rock Cycle. Erosion breaks down and moves rocks. This function makes rainwater essential to the formation of gems and minerals in the crust.

Water filters through the ground, and picks up acidity as it travels. Add some heat or get the right chemical mix, and it becomes corrosive, giving the travelling water the capability to dissolve rocks and minerals. At some point, the water can’t carry anything else, causing it to leave deposits behind in cracks and pockets it passes through. Erosion also uncovers gemstones formed deeper which were brought to the surface from volcanic eruptions.

If conditions are right, water may mix with just the right ingredients to cause a chemical reaction. This type of mineral creation is how gems such as azurite, malachite, opal, and turquoise are formed.

Lionel Herget Turquoise

Gems Formed in the Earth’s Mantle

We only have a limited knowledge of the Earth’s mantle, but evidence tells us that at least two gemstones are formed there: diamond and peridot. This is because they need extreme heat to crystallize. Geological studies have brought scientists to believe that the initial formation of peridot occurred somewhere between 20 and 55 miles underground, and diamonds from even deeper, between 110 and 150 miles underground. These new formations in the mantle would have been melted and destroyed if they weren’t brought to the surface by a swift, powerful eruption.

Volcano Diagram
photo from bfwest01 on Glogster

Types of Gemstone Deposits and Formations

There are various types of deposits and formations of gemstones (precious and semi-precious stones). The most important are as follows.

Magmatic Crystallization

As magma cools, it produces crystals of various minerals. Like diamond, some minerals are gemstones brought to the surface by eruption. Other examples of this type include moonstonetopaz, and corundum (like ruby).

Pegmatite

Near the end of magmatic crystallization, a thin, silicate liquid is left over. If this liquid is pressed into the rock surrounding it, pegmatites are formed with very large crystals, including gemstones such as: berylquartz, spodumenetourmaline, and topaz.

Vein Deposits

After pegmatites have crystallized, the remaining material is a hot water (hydrothermal) mix of chemicals. This material seeps into fissures and cracks in rocks and solidify in Veins of minerals like benitoiteemerald (green beryl)red beryl, and topaz.

Placer Deposits (Gem Gravel)

Minerals with a higher hardness can withstand the normal wear and tear of the world, and come out unscathed when the rest of the rock around them erodes away. Frequently, these are left behind in riverbeds and collect into gravels as found in Sri Lanka, Myanmar (Burma), India, Montana, and other locations around the world. These gravels include precious and semi-precious gemstones including: chrysoberyl, diamond, garnetquartz, rubysapphire, and zircon.

Chemical Precipitates

Malachiteopalrhodochrosite, and turquoise are all chemical precipitates, formed when liquid chemical solutions meet with a catalyst. When the two meet, a reaction takes place, and a solid is created. This solid is called a precipitate. For example, turquoise is formed when an acid containing copper meets volcanic rock containing aluminium and phosphorous.

Metamorphic Deposits

Corundum, garnet, and kyanite are examples of what happens to rock that undergoes high temperature and/or pressure. The composition of the base material is reorganized, forming a new material.