A mineraloid is a natural substance that shares similarities with minerals but doesn’t meet every official, geological requirement to be a mineral. Mineraloids are often organic substances and notably lack a crystal structure, but they’re commonly used as gemstones.
Most people know gemstones as beautiful, often colorful and sparkling treasures, but what is a gemstone? Definitions vary, but many sources describe gemstones as minerals used for adornment, sometimes mentioning organic gemstones.
Why does knowing the difference between minerals and mineraloids matter? If you’re studying gemology, knowing the different properties helps with identification. For casual gemstone lovers, this knowledge can help you shop smarter, practice proper gemstone care, and know the fascinating history behind your favorite jewels.
This guide will teach you everything you need to know about mineraloids vs. minerals and which gemstones are actually mineraloids.
First, we have to establish some base knowledge about minerals.
Every mineral is broadly classified by its chemical composition and internal structure. Some can have the same composition but different internal structures, making them different minerals. For example, diamond and graphite are both made of carbon atoms arranged differently.
In geology, the mineral definition has five parts, or requirements to be met:
Formed naturally (not man-made)
Solid (not liquid or gas)
Inorganic (not formed by or made from living organisms)
Structure of organized, repeated atom patterns that forms crystals (put simply: crystallinity)
Definite chemical composition (not accounting for inclusions)
The last two may be a bit confusing. Let’s do a quick science lesson to explain.
Breaking it down to basics, we start with one or more elements, like silicon or oxygen for example. When elements combine through a chemical reaction, we get compounds (or substances) like silicon dioxide.
In minerals, the elements combine in the same, ordered arrangement every time that mineral forms. For example, every quartz mineral has the same arrangement of silicon and oxygen (silica) inside.
Inclusions that sneak in during formation can create different colored versions of the same mineral. For quartz, inclusions like iron and aluminum can create citrine or amethyst.
You can imagine inclusions like extra flavors added to a cake. Cakes have a set ratio of key ingredients that are essential to it coming out right, like a mineral’s chemical elements. Added ingredients like nuts, fruits, or chocolate are equivalent to inclusions in a mineral.
Clearly, there are lots of definitions and terms pertaining to minerals, but where do all these terms come from?
The official terminology for minerals comes from the International Mineralogical Association (IMA). The IMA was formed in 1958 in part to create standard, consistent names and definitions for minerals.
Founded in 1958, the IMA is relatively modern, started almost 30 years after gemology began. The organization recognizes new minerals annually, having approved a total 5,780 minerals as of January 2022.
Now that you know the requirements of a mineral, what is a mineraloid by simple definition?
Mineraloids are mineral-like, but don’t meet every mineral requirement. As such, mineraloids can vary a bit more than minerals.
However, all mineraloids (except pearl) are amorphous, meaning they don’t have a crystal structure. Additionally, most mineraloids form under similar conditions: low pressure, low temperature, and close proximity to Earth’s surface.
The difference between minerals and mineraloids gets tricky for certain minerals, though — namely, biogenic minerals.
Something is “biogenic” when it’s made of an organism (like plants or animals) or by an organism. Biogenic minerals are similar to other minerals but formed by organisms.
Many minerals are silicates formed in rocks, but some organisms like algae and sea sponges produce silica or incorporate it into their structure. The resulting silicate is a biogenic mineral.
What about rocks? Rocks aren’t minerals either, but rather combinations of multiple minerals and/or mineraloids. Plus, rocks can be gemstones, too! Some examples are tiger’s eye and lapis lazuli. You might see minerals intermixed throughout a rock, forming a “matrix” gemstone, or filling the hollow inside of a rock (e.g. geodes).
As you can see, there’s quite a bit of cross-over to keep track of. Next, we’ll make things a bit more organized by going over the general categories of mineraloids.
Image: ancient Roman glass
There aren’t official classifications of mineraloids, but most fall under one of four categories:
Glass: Hard, inorganic solid with random atom arrangement and no crystal structure (often from cooling too quickly)
Animal-Based: Formed from parts of an animal (shell, skeleton, tissue, etc.)
Plant-Based: Made from secretions (e.g. tree sap) or parts of a plant, often that have fallen off
Liquid: Fluids with definite composition that can crystallize into minerals at certain temperatures
The animal-based and plant-based categories can be combined into the broader “organic” category. The liquid category is small, only covering two substances.
Our categories above don’t cover every mineraloid, though. Opal, for instance, isn’t organic, liquid, or glass, so why is opal a mineraloid? It lacks a crystal structure and has an “n” in its chemical formula, meaning part of its composition (amount of water) varies from one opal to the next.
Each category can also have different subtypes. Glass may be volcanic glass or impact glass (formed from meteorite impact), to name a couple. Animal-based mineraloids may be created by the animal (e.g. pearls) or from their remains (e.g. ammolite).
Below, we’ll go over each mineraloid, including which category of mineraloid (if any) it falls into.
Amber: Plant-based; Fossilized plant resin excreted from ancient pine trees that hardens into a translucent, natural plastic, often with insects or plants inside.
Ammolite: Animal-based; Iridescent, fossilized shell of the extinct mollusk ammonite, composed of calcium carbonate and trace minerals.
Coral (Precious Coral): Animal-based; Often brightly colored, branching, exoskeleton shed by coral polyps and composed of calcium carbonate.
Jet: Plant-based; Type of coal formed when decomposing wood is buried and fossilized under extreme heat and pressure. Is coal a mineraloid, then? Yep!
Libyan Desert Glass: Glass; Impact glass formed in sandy areas when the extreme heat and pressure of a meteorite impact melts surrounding soil and rocks into liquid, which cools too rapidly to form crystals.
Mercury: Liquid; Classified by some mineralogists as a mineraloid for its definite chemical composition and ability to crystallize into a mineral at -38.8℃.
Obsidian: Glass; Dark, volcanic glass formed when flowing lava cools too rapidly for crystals to form; Called “apache tears” when rounded and pebble-like.
Opal: Amorphous, hardened, hydrated silica gel with varying levels of water.
Pearl: Animal-based; Crystalline, organic mineraloid composed of concentric layers of calcium carbonate (nacre) and formed by certain mollusks when irritants get into its shell.
Pumice: Glass; Lightweight, porous, volcanic glass formed when lava explodes out and the rapid decrease in temperature and pressure creates bubbles that freeze in place as it solidifies.
Tektite: Glass; Group of impact glass mineraloids (including moldavite) formed when meteorite impact explosion causes material to rapidly melt, fly into the air, then fall back to Earth, cooling too quickly to form crystals.
Tuff: Rhyolitic volcanic ash compacted together from pressure, containing glassy shards and a crust of other mineral crystals.
Water: Like mercury, a liquid classified by some mineralogists as a mineraloid for its definite chemical composition and ability to crystallize into a mineral (ice) at 0℃
Wait, is ice a mineraloid? Nope, it’s a mineral! It may be surprising, but ice that forms naturally on its own (not in your freezer) is technically a mineral!
After all that, you’re set to crush a mineraloid vs. mineral quiz, that’s for sure! More importantly, you have the knowledge to become a confident, savvy gemstone buyer.
Mineraloids may not be as straightforward or abundant as minerals, but that doesn’t make them any less beautiful as gemstones. Plus, many mineraloids can offer us a connection with the vast and fascinating history of Earth — and the cosmos beyond!
Those looking for a wholly one-of-a-kind gem with a story to tell will certainly fall in love with any mineraloid gem.
Want to explore the array of beautiful mineraloids and minerals? Find your favorite gems today!.
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