Onyx

Key takeaways

Onyx belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Onyx to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Onyx from memory before moving on to harder problems.

Reference excerpt

Onyx is a typically black-and-white banded variety of agate, a silicate mineral. The bands can also be monochromatic with alternating light and dark bands. Sardonyx is a variety with red to brown bands alternated with black or white bands. The name "onyx" is also frequently used for level-banded (parallel-banded) agates, but in proper usage it refers to color pattern not band structure. Onyx, as a descriptive term, has also been incorrectly applied to parallel-banded varieties of alabaster, marble, calcite, obsidian, and opal, and misleadingly to materials with contorted banding, such as "cave onyx" and "Mexican onyx".

Etymology Onyx comes through Latin (of the same spelling), from the Ancient Greek ὄνυξ (onyx), meaning 'claw' or 'fingernail'. Onyx with pink and white bands can sometimes resemble a fingernail. The English word "nail" is cognate with the Greek word.

Varieties

Onyx is formed of chalcedony bands in alternating colors. It is cryptocrystalline, consisting of fine intergrowths of the silica minerals quartz and moganite. Its bands are parallel, unlike the more chaotic banding that often occurs in agates. Sardonyx is a variant in which the colored bands are sard (shades of red) rather than black. Black onyx is perhaps the most famous variety, but it is not as common as onyx with colored bands. Several artificial treatments have been used since ancient times to produce the black color in "black onyx" and the reds and yellows in sardonyx. Most "black onyx" on the market is artificially colored.

Imitations and treatments The name has also commonly been used to label other banded materials, such as banded calcite found in Mexico, India, and other places, and often carved, polished, and sold. This material is much softer than true onyx and more readily available. The majority of carved items sold as "onyx" today are this carbonate material. Artificial onyx types have also been produced from common chalcedony and plain agates. The first-century naturalist Pliny the Elder described these techniques used in Roman times. Treatments for producing black and other colors include soaking or boiling chalcedony in sugar solutions, then treating with sulfuric or hydrochloric acid to carbonize sugars which had been absorbed into the top layers of the stone. These techniques are still used, as well as other dyeing treatments, and most so-called "black onyx" sold is artificially treated. In addition to dye treatments, heating and treatment with nitric acid have been used to lighten or eliminate undesirable colors.

Geographic occurrence Onyx can be found in various regions of the world, including Mexico, Greece, Yemen, Uruguay, Argentina, Australia, Brazil, Canada, China, Czech Republic, Germany, Pakistan, India, Indonesia, Madagascar, the UK, and various states in the US.

Historical use

It has a long history of use for hardstone carving and jewelry, where it is usually cut as a cabochon or into beads. It has also been used for intaglio and hardstone cameo engraved gems, where the bands make the image contrast with the background. Some onyx is natural but much of the material in commerce is produced by the staining of agate. Onyx was used in Egypt as early as the Second Dynasty to make bowls and other pottery items. Use of sardonyx appears in the art of Minoan Crete, notably from the archaeological recoveries at Knossos. Brazilian green onyx was often used as plinths for art deco sculptures created in the 1920s and 1930s. The German sculptor Ferdinand Preiss used Brazilian green onyx for the base on the majority of his chryselephantine sculptures. Green onyx was also used for trays and pin dishes—produced mainly in Austria—often with small bronze animals or figures attached. Onyx is mentioned in the Bible many times. Sardonyx (onyx in which white layers alternate with sard—a brownish color) is mentioned in the Bible as well. Onyx was known to the ancient Greeks and Romans. The first-century naturalist Pliny the Elder described both types of onyx and various artificial treatment techniques in his Naturalis Historia. Slabs of onyx (from the Atlas Mountains) were famously used by Mies van der Rohe in Villa Tugendhat at Brno (completed 1930) to create a shimmering semi-translucent interior wall.

Culture The ancient Romans entered battle carrying amulets of sardonyx engraved with Mars, the god of war. This was assumed to bestow courage in battle. In Renaissance Europe, wearing sardonyx was assumed to bestow eloquence. A traditional Persian assumption is that it helped with epilepsy. Sardonyx was traditionally used by English midwives to ease childbirth by laying it between the breasts of the mother.

Illustrations

Onyx: Black onyx with white streaks — Black onyx with white streaks

Worked examples

Example 1 — a first encounter with Onyx

Start with the simplest possible case. Write down what Onyx claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Onyx before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Onyx ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Onyx

In research: Onyx appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Onyx in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Onyx is common in secondary-school and first-year university syllabi. It links to neighbouring topics Chalcedony, Quartz gemstones, Shades of black, so understanding it makes those chapters shorter.
In everyday life: Look for Onyx outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Onyx in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Onyx means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Onyx out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Onyx in simple terms?

Onyx is a typically black-and-white banded variety of agate, a silicate mineral. The bands can also be monochromatic with alternating light and dark bands.

Why does Onyx matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Onyx?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Onyx.