Duncan is a gems enthusiast with five years of experience dealing with gemstones. He is glad to share some been-there help concerning gems.
Where and how are gems formed?
The majority of gemstones form naturally within the earth, but a good number of them form crystals. That process is referred to as a crystal system where solid atoms arrange in highly repeating patterns. Gemologists learn crystallization and geological ways involved in gemstones formation to help them analyze some of the properties in gemstones.
Environment under the Earth
We will look at the environment under the earth that leads to the formation of crystals and gems. The earth's crust is approximately 4.8 km (3 miles) under the seabed to seemingly around 40 km (25 miles) below continents. Beneath the crust is a mantle around 2993 km (1860 miles) stocky.
The mantle comprises approximately 83% of the earth's volume, which contains magma. If magma comes on the earth's surface, it is called lava. The heat currents keep the mantle in constant motion because it is the hottest near the earth's center.
When the mantle and crust meet in a tumultuous zone of immense pressure and temperatures, many plates form crust and float on top of the liquid blanket.
The process creates a teeter motion that causes some substances to be pushed up in mountains or hills as some go downwards. The magma flows in constant motion with its pressure and movement action on the bottom of the crust. That creates fracturing, which causes rocks to break from the crust and flow with the magma fluid.
Rock melting occurs that impacts remarkable changes in the chemistry of the adjacent magma. This process leaves smaller particles that will include in the gem henceforward. The magma escaping through the fractured crust finds suitable conditions (time and space) to cool and form precious gemstones and crystals.
Basics of Mineral Crystallization
There are five crucial requirements for fling crystallization to happen, namely:
Where to find gems in nature.
Gemologists mimic four ingredients (time being excluded) and make counterfeit crystals that are no different from natural ones. Crystals form under immense temperatures because a solution can hold other minerals in suspension.
More so, when a drop in temperature occurs, the level of solid ingredients a solution can hold in suspension also drops, resulting in crystals' formation. That is to say, a solution containing different minerals will crystallize at different temperatures. For instance, corundum could crystalize first, and when the cooling resumes, topaz will follow and then quartz.
Time is a paramount factor in crystal formation since it cannot be mimicked. But, despite that, it can be interrupted by a couple of factors. This means that passages often open and collapse under the earth. For example, a crystal would form, and the gallery that brings the mineral-rich fluid to the cavity close. This act will cause a stop in the growth of crystals.
And when the passage reopens up, the growth resumes impacting notable effects on a crystal that can occasionally be detectable and sometimes not. Such results include color zoning, twinning, parting, more-in-one development, significant inclusions, strains, etc.
- Color Zoning
Every so often, the layers that succeeded in developing might contain different chemical compositions, resulting in color zoning in a stone. Color zoning is when different colors are, consequently, seen split up in a crystal.
Twinning is when new layers in a stone develop divergent positioning. It is a common feature in corundum.
Parting is when new layers of a crystal can't bond. This phenomenon is common in quarts.
At times when a closed passage reopens and the cavity reaccesses the fluid, a disparate mineral is likely to form on the existing mineral. This process can be seen evidently when a new deposit is being opened since different conditions generate different mineral crystals inside of a cavity.
- Inclusions in a gem are caused by the changes of condition that occur in the cavity. This act may propel a new crystal to grow over an older and larger crystal which stops its growth. If the older crystal gets another chance to be exposed to the growth conditions, then it will grow over the newer one.
It is also possible for two or more crystals to crystallize simultaneously, and if one gains faster momentum of growth, it will overrun the newer one.
Some crystals form as deep as 70-120 miles under the earth, where the pressure is enough to compress them beyond their standard size. The pressure makes the stone vulnerable to rupture.
Crystal formation types
Geological Process: Gems Formation
Gemologists mimic conditions that happen underground and make much more alike crystals.
We are taught only three rock formation procedures in schools, but gemology extends it to four. The familiar rock formations are Igneous rocks created by heat deep in the earth.
Metamorphic rocks are the formation of rocks due to heat and temperature changing existing rocks into something different. And lastly, sedimentary rocks form as a result of deposits from sediments.
However, gemology explains rock formation as a four process incident— molten rock and related fluids, environmental changes, surface water, and gem formation in the earth's mantle.
Magma Crystallization (Molten Rock)
Many minerals form out of magma since it contains many different elements that blend to turn into minerals after cooling down. Changes in temperature, chemistry, and pressure result in the formation of other minerals because there is a change in the available ingredients each time a certain mineral forms.
That is since ingredients are dragged into crystals. Nevertheless, if the conditions available aren't perfect for magma to cool, it will turn into what gemology terms aggregates: tiny particles of interconnected crystals.
Phenocryst is a clear visible crystal found within an igneous rock different from a further fine-grained groundmass. The phenocrysts form in lava between the time magma takes to reach a surface with suitable cooling conditions to grow the fine-grained mineral matrix. Geologists term such a rock as porphyry, an igneous rock containing crystals in a fine-grained groundmass.
Hydrothermal Deposits refer to the cracks in a rock that contains minerals caused as a result of underground water or hot fluids that are associated with magma. When hydrothermal fluids move through the fractures in the rock, they come in contact with other minerals and substances, and when the right conditions such as temperature, time, pressure and space combine, then crystals form.
Pegmatites are crystalline igneous rock containing roughly grained crystals and usually long. They occur as heaps containing volatiles in fractures of the igneous rocks, which cools to form pegmatites.
Magma/lava cooling off
Changes in environment affect gems formation
Changes in temperatures and pressure can cause minerals to become unstable, which results in some minerals transforming without melting, a process known as metamorphism. Metamorphism is categorized into two namely: contact and regional metamorphism.
Regional metamorphism is when two or more continental plates (earth surface pieces) complete for space while they flow on the mantle, and one of them is pushed upwards and another down. Intensive heat and pressure occur during this process due to the two landmasses coming in contact. When temperatures are high enough to melt the rock, minerals change into new kinds.
Contact metamorphism is when an existing rock melts after magma forcefully enters it under intensive heat. The rock recrystallizes, and new species are formed.
Water is a crucial factor in mineral recycling because rocks and other minerals are transported to new destinations where they break and mix with the available chemical substance leading to the formation of new gems.
Formation of Fossils
Sometimes running water comes in contact with lots of new acids that can make it acidic. Chances are, the water can turn to be caustic if it meets the appropriate conditions. For instance, gems like malachite, opal, azurite are formed due to water combining with other minerals that form chemical reactions, and deposit the dissolved minerals as new minerals in cavities and seams.
Also, fossils form due to water becoming oversaturated due to mixing with lots of components; it leaves the surfeit pores and cracks of hosting rocks.
Acidity in water
Heat is important for crystals to form
Gems Formed in the Earth's Mantle
Gems that form in the earth's mantle go through excessive heat. According to gemology, gems of these kinds crystallize under tremendous temperature. Due to explosive eruptions, they are brought up on the surface or near the earth's exterior. Diamond and peridot gems are examples of this kind of gems, and they form from as deep as 20-250 miles below the crust.
© 2021 Duncan Nyabaro