A stone specification is the point at which a material stops being a mood-board image and becomes a contract. Almost everything that goes wrong with natural stone on site — the wrong shade arriving, slabs that will not cover a lobby without an obvious join line, a floor that is beautiful and lethal when wet — can be traced back to a specification written in two lines when it needed twenty.

This guide is for architects, interior designers, main contractors and developers working on villa, hotel and commercial projects in Oman and the wider Gulf. It sets out what a natural stone specification needs to contain, in the order the decisions should actually be made, and it flags the clauses that get quietly interpreted in the supplier’s favour when they are left vague.

What a stone specification is actually for

A good specification does three jobs at once. It tells suppliers exactly what to quote, so that three quotations can be compared on the same basis instead of on headline price alone. It tells the installer what is acceptable and what is not, so that a disagreement on site has a written answer rather than a louder voice. And it protects the design intent through the months between approval and delivery, when the person who originally chose the stone is no longer in the room.

It follows that a specification is not a description of the stone you liked. It is a description of the boundaries within which the delivered stone must fall. Natural stone is geological rather than manufactured, so it always has a range rather than a single value — and a specification that ignores that fact is a specification that will be argued about.

Start with the application, not the stone

The most common failure in stone specification is choosing the material first and discovering afterwards what it has to survive. Reverse the order. For each area, write down what the surface will genuinely face: the volume and type of foot traffic, water and its chemistry, heat, oil and acid exposure, direct sun, abrasive sand carried in from outside, and whether anyone will walk on it barefoot.

An external pool surround in Muscat has to stay tolerable underfoot in summer, drain quickly, and cope with chlorinated water and salt-laden air. A hotel reception floor has to take rolling luggage and daily machine cleaning. A bathroom vanity top will meet acidic cosmetics every morning. These are different engineering problems, and they point towards different stones long before anyone opens a sample box. Our guide to the best stone for high-traffic areas works through that reasoning in detail.

Step 1 — Name the material so it cannot be substituted

Commercial names are marketing, not geology. The same block can be sold under three names in three countries, and two entirely unrelated stones can share one name. If a specification says only “beige travertine”, a supplier is free to deliver anything beige and still claim compliance.

Name the material on four levels: the commercial name, the petrographic family (travertine, granite, marble, onyx or crystal), the country and quarry of origin where it is known, and — the line that does the real work — the block or lot that the approved sample came from. Then add an explicit clause: no substitution without written approval of a physical sample. Colour and veining vary from block to block in every natural stone on earth, so tying the order to an approved lot is the only reliable way to control what arrives at the port.

Step 2 — Fix the format, thickness and tolerances

State the nominal size of each element, the thickness, and the tolerance you will accept on both. Twenty millimetres is a common interior floor and worktop thickness; eighteen and thirty millimetres are equally standard; external paving and heavy-traffic floors usually move thicker. The number matters less than the fact that you have stated it, because thickness drives price, weight, structural load, fixing method, and every skirting and threshold detail downstream.

Add tolerances explicitly — on length and width, on thickness, on squareness and on flatness — and state whether calibration is required. Say whether the back face may be resined or mesh-backed, and whether resin and filler are acceptable on the visible face. Travertine is routinely supplied either filled or unfilled, and the choice changes the appearance, the cleaning regime and the price. If it is not written down, you will get whichever the factory finds easier.

Step 3 — Specify the finish, and say what it must not be

Finish affects performance as much as appearance. Polished surfaces are the most reflective and the easiest to wipe clean, but they show scratches and etch marks and become slippery when wet. Honed surfaces are matt, more forgiving of wear, and slightly more absorbent. Leathered and brushed finishes add texture and disguise fingerprints and water marks, which is why they suit dark stones and outdoor areas. Bush-hammered, flamed and sandblasted finishes are coarser still and belong outside.

Specify the finish for each face — top surface, exposed edges, and any visible underside — and specify the edge profile as a separate line. If you have a preference between honed and leathered, also state what is not acceptable, because “matt” means different things in different factories. Our comparison of polished, honed and leathered finishes sets out where each one belongs.

Step 4 — Slip resistance, and where it genuinely matters

Any floor that will be wet — a pool surround, a bathroom, a spa, a hotel entrance during the khareef or a rain shower — needs a slip resistance requirement written into the specification rather than assumed. Several test methods are in use internationally, including pendulum, ramp and DCOF methods, so name the method as well as the value, and ask the supplier to confirm which finish was tested.

Never accept a slip resistance claim made for “the stone”. Slip resistance is a property of the finished surface, not of the material, and the same stone will perform very differently polished, honed and flamed. It also changes over time: heavy polishing during maintenance, some sealers and certain cleaning products all reduce it. Where a surface is safety-critical, specify the test, the finish and the maintenance products together.

Step 5 — Decide the fixing method before you fix the thickness

Adhered installation, mechanically anchored cladding, pedestal-mounted external paving and traditional mortar beds each impose different demands on the slab. Mechanical anchoring needs enough thickness to take a kerf or anchor and requires the stone’s flexural strength to be considered. Adhered wall stone has a practical weight limit per square metre and needs the correct adhesive class. Pedestal paving needs consistent calibration and usually greater thickness.

Write the fixing method into the specification alongside the joint width, the movement joint layout, and who is responsible for setting out. A large-format floor with a one-millimetre joint is a different and far less forgiving product from the same floor with a three-millimetre joint, and the difference has to be agreed before the stone is cut, not after it has arrived. Dense materials such as those in our granite collection are often specified for exactly these demanding fixing conditions.

Step 6 — Sealing, grout and cleaning clauses

Say whether the stone is to be sealed, with what type of product, at which stage of the installation, and by whom. Impregnating sealers reduce absorption without changing the appearance; topical sealers change the look and wear off unevenly under traffic. Both need re-application eventually, so the specification should state an expected maintenance interval and hand that information to the facilities team rather than leaving it to be discovered.

Grout deserves a clause of its own. Grout colour changes the perceived colour of a floor far more than most people expect, and the wrong grout can permanently stain the edges of a light stone. Specify grout type, colour and joint width, and require a grout sample applied to the approved stone rather than judged from a chart. Finally, name the cleaning products that are prohibited — acidic cleaners on marble, travertine and other calcareous stones being the obvious and most frequently ignored example.

Step 7 — Samples, range samples and mock-ups

One sample is not a specification. Because natural stone varies, ask for a range sample: two or three pieces showing the lightest, the darkest and the most heavily veined material the supplier is prepared to deliver. Everything inside that range is acceptable; everything outside it is not. That single clause prevents most colour disputes before they start.

For any significant area, require a mock-up — an assembled panel or a small floor bay, built with the specified fixing method, joint width, grout and finish, and viewed under the actual lighting of the space. Approve it in writing, keep it protected on site, and make it the reference for acceptance of the delivered material. Where slabs are being selected for a feature wall or a book-matched surface, they should be chosen and photographed at the yard before cutting begins. Our checklist of what to check before buying a stone slab works well as an annexe attached to the specification.

Step 8 — Quantities, wastage and attic stock

Specify quantities as a net area plus a stated wastage allowance rather than as a single number, and say clearly who carries the risk if that allowance is exceeded. Wastage depends on the cutting layout, pattern matching, the number of penetrations and the size of the elements. A plain rectangular floor and a book-matched lobby with a border are not the same job and should not carry the same allowance.

Always specify attic stock: a percentage of the order, taken from the same lot, delivered and stored on site for future repairs. Stone cut from a different block years later will not match, and this clause costs very little at order stage compared with the alternative. If budget pressure is shaping these decisions, our note on what affects the price of natural stone explains which specification choices move the number most.

Step 9 — Packing, delivery and site storage

Stone is heavy, vulnerable at the edges and easy to stain before it is ever installed. Specify crating, edge protection, interleaving material, and the marking of crates by area and installation sequence. State the delivery programme against the construction programme, and state access and offloading requirements — crane or forklift capacity is a genuine constraint on many Muscat sites and a common cause of demurrage.

Add a storage clause as well: crates upright on timber bearers, off the ground, under cover, out of direct sun and well away from wet trades. Cardboard and some plastic sheeting will stain light stone once they get wet. It is a short paragraph that prevents an expensive argument.

A worked example: a villa entrance hall in Muscat

Rather than offer a template to copy, it is more useful to see how the decisions chain together for one real area. The brief is a double-height entrance hall, air-conditioned, highly visible, with sand carried in from outside and daily cleaning.

The application points towards a dense, low-absorption stone with good scratch resistance, which narrows the field towards granite and the harder marbles. The daily grit argues for honed rather than polished, because a polished surface in a sandy environment will show traffic patterns within the first year. The double height and the long sightline argue for large-format pieces with narrow joints, which in turn demands calibrated slabs and a genuinely level substrate. Large format on a busy floor then pushes the thickness up and points to adhered installation over a screed with movement joints at correct centres.

The specification lines that follow almost write themselves: named stone and approved lot; 900 × 900 mm nominal; 20 mm thick; honed top with eased exposed edges; stated tolerances on thickness and flatness; a three-piece range sample; a 1200 × 1200 mm mock-up bay built in situ; grout colour approved on the stone itself; impregnating sealer applied before grouting; a stated wastage allowance calculated from the cutting layout; attic stock from the same lot; and crates marked by bay in installation sequence. Every one of those lines exists because a decision earlier in the chain forced it.

Five specification mistakes that cost real money

Specifying by photograph

A render or a phone photograph of a slab is not a range. Photographs flatten contrast, shift colour under different lighting, and cannot show how the stone reads across twenty square metres. Approve physical material, in the light of the actual space where possible.

Leaving thickness out

It is the single line that changes weight, price, fixing method and detailing, and it is the one most often omitted. A specification without a thickness is an invitation to quote the cheapest option.

Approving a sample without approving a lot

The sample was honest. The next block from the same quarry simply looks different, because that is what quarries do. Without a lot reference, nobody has broken any rules and you still have a floor you did not choose.

Specifying a finish for appearance alone

A polished pool coping is a specification error rather than a design choice. Finish has to answer to safety and maintenance before it answers to the mood board.

Omitting attic stock

Three years later, a cracked slab in the middle of a hotel lobby cannot be replaced invisibly if no matching material was retained. A few square metres in a store room is the cheapest insurance in the whole package.

How to brief a supplier so the quotation matches the specification

Send suppliers the same information you want back from them: an area schedule with net quantities, sizes and thicknesses, the finish for each element, the fixing method, programme dates, delivery location and access constraints, sample and mock-up requirements, and the substitution clause. Ask for the quotation to be broken down by area and by element rather than presented as a single figure, and ask for lead time and block availability to be stated separately from price.

That is also the moment when a supplier can tell you something genuinely useful — that a size you have specified produces heavy wastage from the standard block dimensions of that stone, that a finish you want is available on one candidate in your shortlist but not another, or that a lead time will not meet your programme. Those conversations save money when they happen before tender rather than after award.

Frequently asked questions

What thickness should I specify for a stone floor?

There is no universal answer. Twenty millimetres is a common interior default; thinner material is used where weight matters and the substrate is sound; thicker material is normal for external paving, heavy traffic and large formats. Decide thickness together with format and fixing method, then confirm it with your supplier against the block sizes actually available for the stone you have chosen.

Can I specify natural stone from a photograph or a rendered image?

Not safely. Images cannot convey the variation across a block, and screen and print reproduction shift colour. Use images to shortlist, then approve physical range samples and a mock-up before any material is cut.

What is a range sample, and why does it matter so much?

It is a set of pieces showing the extremes of colour and veining the supplier will deliver — lightest, darkest and most heavily figured. It converts an unwritten expectation into an agreed, inspectable limit, and it is the clause that resolves most colour disputes before they reach a claim.

Should the specification name a slip resistance value?

Yes, for any surface that will be wet, and name the test method alongside the value. Also record which finish was tested, because slip resistance belongs to the finished surface rather than to the stone as a material.

How much wastage should I allow?

It depends entirely on the cutting layout, element size, pattern matching and the number of cut-outs. Rather than applying a habitual percentage, ask the supplier to calculate the allowance from your actual layout, and state in the specification who carries the risk if it is exceeded.

Who should approve the mock-up?

Whoever will ultimately be held to the design intent — normally the architect or lead designer, with the client and the main contractor present. Approve it in writing, photograph it, and keep it on site as the physical benchmark for accepting delivered material.