Mineral Classes Explained
Silicates, oxides and how crystals are sorted by chemistry, and what it means for hardness.
Minerals are classified chemically using the Dana system, grouped by their dominant chemical component into classes including silicates, oxides, carbonates, sulfides, halides and more. Silicates are the largest class by far and include most crystals sold as jewellery.
Two Ways to Classify a Crystal
There are two separate ways mineralogists sort crystals, and it's worth knowing the difference, because they answer different questions. Crystal system is about geometry: the internal symmetry that determines shape. Mineral class is about chemistry: what a mineral is actually made of. This article is about the second one.
The Dana System
The standard framework for chemical classification is the Dana system, created in the 1830s and 40s by the American mineralogist James Dwight Dana. It sorts minerals into classes based on their dominant chemical component, usually a specific negatively charged ion or ion group, called an anion.
Despite being nearly two centuries old, it's still the reference system used today, refined many times over but structurally unchanged at its core.
The Major Mineral Classes
Silicates are the largest class by a wide margin, making up the majority of the Earth's crust. Every silicate is built around a basic unit of one silicon atom bonded to four oxygen atoms, arranged in different ways: isolated, in chains, in sheets, or in three-dimensional frameworks. Quartz, and by extension amethyst and citrine, are silicates.
Carbonates contain a carbonate ion, built from carbon and oxygen. Malachite is a carbonate, and so is calcite, the mineral used as reference point 3 on the Mohs hardness scale.
Oxides are minerals where oxygen bonds directly to one or more metallic elements. Oxide minerals tend to be relatively hard and durable.
Halides contain a halogen element: chlorine, fluorine, bromine or iodine. Fluorite is the best-known example.
Sulfides pair a metal with sulfur. Pyrite is the classic example.
Native elements are minerals made of a single element in pure, uncombined form, gold and copper being the most familiar.
Hardness and Chemistry: The Mohs Scale
Hardness is measured using the Mohs scale, a ten-point scale created in 1812 by the German mineralogist Friedrich Mohs, ranking minerals by their resistance to scratching. It runs from talc, at 1, to diamond, at 10, and it's directly connected to chemical class, because hardness itself comes down to the strength of the chemical bonding between atoms.
For everyday reference, a fingernail sits at around 2.5, a copper coin at around 3.5, and a steel knife blade or window glass at around 5.5.
Why This Matters for Care
This is exactly why our care guidance treats stones differently depending on what they are. Selenite sits at roughly 2 on the scale, barely harder than a fingernail, which is why it scratches so easily. Quartz, at 7, is tough enough for daily wear without much thought.
Mineral Classes, Answered
How are minerals classified chemically?
Primarily using the Dana system, which groups minerals by dominant chemical component, silicates, oxides, carbonates, sulfides, halides, sulfates, phosphates and native elements.
What is the most common mineral class?
Silicates, by a wide margin. They make up most of the Earth's crust and most gemstone-quality crystals.
What is the Mohs hardness scale and why does it matter?
A ten-point relative scratch-resistance scale from talc (1) to diamond (10), created in 1812. It tells you practically how carefully a stone needs to be handled, worn and stored.
Why do some crystals scratch more easily than others?
Hardness comes down to the strength of chemical bonding between atoms, and to some extent atom size. Softer classes tend to have weaker internal bonding than harder ones.