Where curling stones come from
An Olympic sport whose central object is supplied by two quarries, one of which is a seabird reserve where blasting is banned. This is not a colourful fact somebody made up. It is the supply chain.
Last updated 2026-08-18
Two quarries, one sport#
Curling is played in more than seventy countries. The granite it is played with comes from two places on earth, and neither of them is large.
- 2quarries supplying the entire sport
- ~2,000stones a year made by Kays of Scotland
- 60myears old, roughly, the granite itself
- 50–70years a well-kept set is expected to last
This is not a shortage of granite. The world has plenty of granite. It is a shortage of granite with the specific properties a curling stone needs: dense enough to take the weight into a small contact area, tough enough to survive being hit by another stone several thousand times a season, and — the hard one — close enough to non-porous that it does not absorb water.
A stone spends its life on ice. Any water that soaks into the running surface freezes, expands, and breaks the granite from the inside. Most granite does this. Blue hone essentially does not, which is why an unremarkable-looking lump of rock from a bird island is the material of record for an Olympic sport.
Ailsa Craig#
Ailsa Craig is an uninhabited volcanic plug about ten miles off the Ayrshire coast in the Firth of Clyde, roughly two miles around. It is a bird sanctuary, with internationally significant colonies of gannets and puffins, and it is privately owned by the Marquess of Ailsa. Kays of Scotland holds the exclusive right to take granite from it.
The constraints that follow from being a bird reserve are real and they shape the supply:
- No blasting. Granite is collected rather than quarried in the ordinary sense — loose surface material and controlled extraction, not explosives.
- A narrow season. Work happens outside the seabird breeding season, in practice a window in the autumn.
- Everything leaves by sea. There is no harbour worth the name and no road. The granite is barged off.
An extraction of a few hundred tonnes is enough to keep the sport supplied for years, which is why the harvest happens rarely and makes the news when it does.
The granites, and what each is for#
A modern top-flight stone is not one rock. It is at least two, chosen for different jobs.
| Granite | Source | Used for | Why that one |
|---|---|---|---|
| Ailsa Craig Blue Hone | Ailsa Craig, Firth of Clyde, Scotland | The running band — the ring that touches the ice | Almost non-porous, so it takes up virtually no water. Water absorbed into a running surface freezes, expands and destroys it, which is why the part in contact with the ice is the part that gets the rarest stone. |
| Ailsa Craig Common Green | Ailsa Craig, Firth of Clyde, Scotland | The body and the striking band | Tough under repeated impact and resistant to splintering and to heat transfer. A stone is hit thousands of times a season and the body has to survive it. |
| Ailsa Craig Red Hone | Ailsa Craig | Historic | Effectively exhausted. It was quarried away over the years and is no longer a working material. |
| Trefor granite | Trefor, Llŷn Peninsula, north Wales | Whole stones | The other working source, and the reason "all curling stones come from Ailsa Craig" is wrong. Canada Curling Stone has exclusive rights to it and has made stones from it since 1992. Trefor stones were used for every game at the 2002 Salt Lake City Olympics. |
Kays calls the composite construction an Ailsert: a blue hone insert fitted into a common green body, so that the part touching the ice is the rare, non-porous stone and the part absorbing collisions is the tough one. Stones may have a single insert or a double, one on each face, so the stone can be turned over and given a second working life.
The anatomy of a stone#
A curling stone looks like a simple object and is not. Four of its features are deliberate and none of them is obvious from a photograph.
The running band#
The underside is concave. The stone stands on a narrow annulus about 130 mm across and 6 to 13 mm wide, and the centre of the stone never touches the ice at all. This is the single most important design feature: it concentrates 19 kg into a few square centimetres, which is what produces friction predictable enough to build a sport on. It is also, as the physics page explains at length, where the entire argument about why stones curl takes place.
The striking band#
The belt around the circumference, at the height where one stone hits another. It is the part that wears, and it is why the body granite has to be tough rather than merely dense.
The bolt#
A stone is drilled through the middle and the handle is bolted on. This is also how a handle is changed — the colours are the two teams' identification and they are not painted on.
The numbers#
Two kinds of number, and it is worth knowing which is which. The rulebook gives limits; the simulator uses measurements of a representative stone.
| Property | Value | Where it comes from |
|---|---|---|
| Weight, including handle and bolt | 17.24–19.96 kg (38–44 lb) | World Curling rule R2(a) — a permitted range, not a spec |
| Maximum circumference | 914 mm (36 in) | Rule R2(a) |
| Minimum height | 114 mm (4.5 in) | Rule R2(a) |
| Mass used by this simulator | 19.0 kg | A representative stone near the top of the permitted range |
| Radius | 142 mm | Measured |
| Height | 114.5 mm | Measured |
| Running band radius | 65 mm | Measured |
| Coefficient of restitution | 0.94 | How much speed survives a stone-on-stone collision — granite is very elastic |
| Moment of inertia factor | 0.385 | Not the 0.5 of a uniform disc: a stone’s mass is concentrated toward the middle and low down |
A sheet needs sixteen stones — eight per team — and a club with six sheets therefore owns ninety-six of them. This is why stones are the largest single capital item a curling club has, why second-hand and reconditioned stones are a real market, and why a new club almost never buys new ones.
How one is made#
In outline, and it is more hand work than people expect:
- The blank. A cylinder is cored or cut from the raw block, then turned to the profile — the domed top, the striking band, the concave underside.
- The insert. For a composite stone, a recess is machined into the underside and a blue hone disc is fitted into it. This is the part that will touch the ice.
- The running surface. The band is cut to its final width and profile, and textured. This is not polishing — the roughness of the running surface is functional, and at least one theory of why stones curl says it is the functional thing about a stone.
- Drilling and finishing. The through-hole for the bolt, then polishing of the body, then the handle.
- Matching. Stones are checked and grouped so that the sixteen on a sheet behave alike. A set with one stone that runs differently is a set that ruins games.
Kays produces on the order of two thousand stones a year, which is about forty-eight a week — a hundred and twenty-five sets. For the entire world.
How long a stone lasts#
Longer than the people using it, generally. A well-maintained set is expected to last fifty to seventy years, and stones from the nineteenth century still turn up in play at clubs that never had the money to replace them.
What wears out is not the stone but its running surface. Two maintenance operations keep a set honest:
- Reconditioning. A thin layer is machined off the running surface to reach fresh granite, restoring the band's width, shape and dimensions to current specification. This is the only real fix for a set that has become inconsistent — the stones that curl differently from each other are the ones a club complains about for years before doing anything.
- Reprofiling the strike band, roughly every twenty-five years on a busy set.
Between those, the day-to-day care is mostly about keeping the stones off warm floors and out of standing water. A stone that gets wet and then gets frozen is a stone with a shortened life.
Handles, and the one with electronics in it#
The handle is bolted on, is coloured to identify the team, and is otherwise unremarkable — except at the top level, where some are instrumented.
The Eye on the Hog handle exists to settle hog line violations. A stone must be released before the thrower's hand crosses the near hog line, and at competitive speed no human official can reliably see it. The handle contains a magnetic sensor, a tilt switch that wakes it when the stone is tipped to be cleaned, a touch-sensitive grip, and LEDs. It detects the moment the hand leaves the handle relative to a magnetic strip under the ice, and flashes green for a legal release and red for a violation. Its quoted accuracy is about 3 mm at 10 feet per second, which is far beyond what an umpire can judge.
The system originates in a University of Saskatchewan patent, developed commercially by Startco Engineering. It is also the reason the rulebook forbids a glove on the delivery hand when the handles are in use: the sensor needs skin contact. In recent seasons it has fallen out of use at some major events after a run of false positives, and the sport has reverted to the honour system — which is, characteristically, considered a workable answer.
Elsewhere#
- Kays of Scotland — curling stonesThe manufacturer with the Ailsa Craig rights, describing its own process.
- Canada Curling Stone CompanyThe Trefor granite side of the industry, including reconditioning services and the maintenance guidance most clubs work from.
- Ailsa Craig — Team GBA readable account of the island and the harvest.
Sources#
- Primary The Rules of Curling, R2: Stones World Curling, 2025The weight, circumference and height limits, and the rules for damaged and broken stones.
- Curling stones come from two quarries in the world Geography RealmOn Ailsa Craig and Trefor as the two working sources.
- Primary Granite types and stone maintenance Canada Curling StoneOn Trefor granite, the 1992 start of production, the 2002 Olympic use, and reconditioning intervals.
- Eye on the HogOn the sensing handle, its origin in a University of Saskatchewan patent, and its accuracy.
What that running band is doing What you actually need to buy