Agate, quartz and amethyst are the same mineral, silicon dioxide, arranged three different ways. Marble is not related to any of them. That single fact explains most of what follows: why an agate coaster shrugs off a dropped key and a marble one does not, why amethyst comes in points and agate comes in bands, and why no two pieces of either are alike.
Agate is banded chalcedony
Chalcedony is quartz whose crystals are too small to see, packed tight. Agate is the banded kind. It forms inside a cavity in volcanic rock, one layer of silica-rich water at a time, over a very long period, and each layer sets a little differently from the one before it. Those layers are the bands. Cut across them and you get the concentric rings you see on a coaster or a tray. Cut the same stone at another angle and you get sweeping curves instead.
Jasper is the same material without the banding: opaque, even in tone, often red or green. If a piece is chalcedony and has no bands, it is usually jasper.
Quartz is the same mineral with bigger crystals
When silicon dioxide grows slowly with room to spare, it forms crystals large enough to see. That is quartz proper. Clear quartz is colourless. Rose quartz takes its pink from trace amounts of titanium, iron and manganese, and it is usually cloudy rather than clear, which is why it reads as a solid soft pink rather than as glass.
Amethyst is purple quartz
Same mineral again. The purple comes from iron held in the crystal structure, altered by natural radiation over geological time. Amethyst grows as pointed crystals lining the inside of a cavity, which is what a geode is: a hollow rock with a crystal-lined interior. That is why so many amethyst pieces have one raw, spiky face and one cut, polished one. The spiky face is not a finish. It is how the stone actually grew.
Depth of colour varies enormously, from pale lilac to deep grape, and it can vary within a single crystal.
Marble belongs to a different family
Marble is limestone that has been squeezed and heated inside the earth until it recrystallised. Chemically it is calcium carbonate, not silica. It has veins rather than bands, and those veins are other minerals that were in the limestone before it changed.
The practical difference is hardness. On the Mohs scale, which runs from talc at one to diamond at ten, quartz and the whole quartz family sit at seven. Marble sits at around three. Seven is harder than a steel key, harder than grit walked in on a shoe, harder than most of what lands on a coffee table by accident. Three is not. Marble also reacts to acid, so lemon, vinegar and wine will dull the polish where they sit. Agate will not.
None of that makes marble the weaker choice. It makes it a different one, and it is worth knowing which you are buying. Caring for each follows from the same difference.
The other stones you will see here
- Labradorite is a feldspar rather than a quartz, grey through denim blue, and it takes a very smooth polish.
- Fluorite is calcium fluoride. Softer than quartz, and it often carries green, purple and blue in the same piece.
- Petrified wood is wood whose cell structure has been replaced by silica, so the grain of a tree survives inside a stone that behaves like agate.
- Lapis lazuli is a rock rather than a single mineral, deep blue with flecks of gold-coloured pyrite through it.
- Selenite is a gypsum, very soft at around two on the Mohs scale, and it should be kept away from water.
Why no two pieces match
All of this formed over millions of years in conditions that were never repeated exactly. The banding on one agate slab is a record of what was dissolved in the water at that spot, in that order. The next slab cut from the same block will be close but not identical. A slab from another block will not be close at all.
This is why the photograph on a product page shows one example of a stone rather than the piece you will be sent, and it is the reason people buy stone instead of something moulded. If you want two pieces to sit together and read as a set, buy them at the same time from the same range rather than months apart.
What the metal edge is doing
Most of our stone pieces are finished with a metal edge, and that edge is electroformed rather than coated. Electroforming grows real metal onto the stone, bonded to the surface rather than sitting on top of it. A coating can chip, flake or rub away at the rim, which is exactly where a coaster is picked up and put down a few thousand times. Electroformed metal does not. The FAQs go into how the finishes differ.
Choosing by where it will sit
Hardness is the shortcut. Anything that meets keys, glasses, hot cups and other people's hands wants to be in the quartz family: agate coasters, agate trays, quartz and amethyst pieces. Marble is better suited to somewhere calmer, a shelf or a console or a desk, where nothing acidic is going to be set down on it and left.
Colour is the other half of the decision. Amethyst and fluorite are the loud choices and work best where there is little else competing. White agate and marble sit under almost anything. The warm agates, carnelian through honey, do the most in a room that is otherwise cool.
Where to find each
- Agate runs through nearly everything: coasters, trays, clocks, knobs and handles and the side tables.
- Amethyst and quartz appear across the same ranges and in the statement pieces.
- Marble is mostly in Table & Dining.
- Brass, which is a metal rather than a stone, carries the personalised range and the pooja and festive pieces. Cleaning it is a separate matter, covered in how to clean brass at home.