We decorate ourselves with pieces of our planet. Many “stones” used in jewellery are minerals, so let’s return quickly to some geography (in my school we didn’t learn geology, so geography it is). Earth is a planet and like all planets it has a crust, the outermost layer. We walk on it (the continental crust) and we swim on top of it (the oceanic crust).
The crust floats on the mantle, which in turn covers the hot core. The lithosphere consists of rock. Rock and mineral isn’t the same thing. A rock doesn’t have a regular chemical structure and it can be formed from minerals or non-minerals. There are three main types of rock:
- Igneous rocks: are formed with or without crystallization through the cooling and solidification of magma or lava (Lat. ignis = fire). Minerals have a key position in the formation of these rocks.
- Sedimentary rocks: are formed in the sediment layers at the surface of the planet during deposition of materials. Most of these rocks contain very few different kinds of major minerals, and common ones are quartz and calcite.
- Metamorphic rocks: are created through metamorphosis of existing rocks, which can happen for instance due to being under high pressure below rock nearer the atmosphere. Metamorphic minerals form only under those conditions and these so-called index minerals include kyanite and some garnet.
An ore is a rock, which contains minerals with a high concentration of a particular element, typically a metal.
A geode is a geological secondary structure, which occurs in certain sedimentary rocks and volcanic rocks (igneous rocks of volcanic origin). Geodes are highly popular in jewellery at the moment thanks to the sparkly silicates and/or carbonates deposited on the surface.
Gems have so-called ornamental value and they are minerals, either semi-precious stones or precious stones, the latter of which are diamond, ruby, sapphire, and emerald.
Minerals are currently defined as “an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.” In other words, the foundation of classification of minerals is in differences in chemical composition, crystal structure, and physical properties among others.
The majority of minerals are derived from the crust of the Earth and since its two most abundant elements are the metalloid silicon and oxygen, most minerals are silicate minerals with Si and O in different ratios. The rest are in the non-silicate minerals group, the majority of which are very rare. Some common ones are calcite, pyrite, magnetite, and hematite.
Be happy that someone else takes care of mineralogy, as there are over 4900 species. The vocabulary is also rather challenging, so we’ve scratched the surface only (or should I say crust, heh).