For an architect, natural stone specification is far more than choosing a colour, texture, and finish.

The correct specification should link the design intent and project-specific requirements to technical properties and performance of the selected natural stone.

Below is a list of things that architects should consider prior to finalising the specification of natural stone.

1. Start from the Application

The same stone cannot be specified in all areas of the same project.

Lobby floor, façade, staircase, bathroom wall and landscape pathway are different applications which involve distinct conditions for natural stone application.

In this regard, the specification of natural stone should start from the identification of the exact application area and conditions that stone is going to face.

These include:

• Application inside or outside the building
• Installation vertically or horizontally
• Number of pedestrians who are going to step on the natural stone
• Moisture conditions
• Weather conditions
• Cleaning and maintenance
• Requirements for the structure or aesthetics

This approach to stone selection in the Natural Stone Institute’s design guidance covers physical properties, finishes, samples, mock-ups and cost considerations.

2. Specify the Stone, Not Just the Colour

A specification needs to give sufficient information for the stone to be identified and its characteristics assessed.

Instead of a generic description like “beige stone” or “grey marble”, the specification should determine stone variety, desired aesthetics and performance requirements.

Natural stone is a natural product and therefore variations in color, veining, grain and texture are expected. It should be distinguished from undesirable defects.

3. Align the Characteristics with the Application

This is when a specification goes beyond a design document.

According to the intended use, the architect needs to know about:

• Water absorption
• Compressive strength
• Flexural strength
• Resistance to abrasion
• Slip resistance
• Density and specific gravity
• Resistance to stains and chemicals

Of course, not every characteristic is required in every application. According to the Natural Stone Institute, it is important to select tests based on the actual use of the stone rather than apply standardized requirements to each and every application.

For example, tests for abrasion and slip resistance can be important for the floor covering, while anchorage and structural aspects need to be considered for the exterior wall cladding made of natural stone.

4. Don’t Overlook the Finish

Finishes are part of the specification, not just the ultimate aesthetic finish.

Finishes such as polished, honed, flamed, brushed and textured finishes may result in very different outcomes. The right finish is going to depend on location, use, maintenance and desired aesthetics.

For wet or high-footfall locations, it is important to look at the actual performance of the finished product rather than simply assume that one type of finish is going to work in any place.

5. Specify Size, Thickness and Tolerances

The stone specification should include the physical requirements of the project.

This may involve, depending on application:

• Nominal length and width
• Thickness
• Edge profile
• Finish
• Dimensional tolerances
• Jointing requirements

This is especially critical for large projects where hundreds or thousands of pieces will have to fit together in terms of both appearance and installation.

6. Specify the Whole Installation System

Stone selection is only one element of the stone installation design process.

For natural cladding purposes, architects will need to consider the way in which the stone is anchored and supported on the building. Issues like anchorage, substrates, joints, movement and environmental loads can all impact the performance of the installed system.

For instance, ASTM C1242 gives recommendations for exterior dimension stone anchors and anchoring systems selection, design, and installation.

The specification should thus be clear as regards which party is responsible for stone fabrication, anchorage, fixing, and installation design.

7. Refer to Appropriate Standards

If relevant, the specification should refer to the appropriate standards and testing methods instead of making loose reference to high quality and heavy-duty materials.

For Indian projects, relevant BIS standards would include those on limestone, marble, structural granite, and sandstone. There is also an appropriate standard listed by BIS for determining the resistance to wear by abrasion of natural building stones: IS 1706:2024.

Depending on the project and contract terms, for international projects, the relevant standards might also be ASTM or European.

8. Approve Samples and Mock-Ups

While the specification could describe the requirements from the point of view of material properties, there is a need to see the visual result of its use.

Samples could help assess the colour, veining, grains, and finish. Mock-Ups, in their turn, could offer better insight into the combination of stone, joints, finish, and installation details.

Stone samples are used for assessing colour, veining, grain and finish. In case of large-scale projects, mock-ups can be helpful in gaining a better understanding of the combination of stone, jointing, finish and installation details.

The Natural Stone Institute mentions samples and mock-ups among other considerations to make in stone selection guide.

9. Assess Availability Before You Specify

Even the most beautiful stone will not be any good in case you are not able to acquire the necessary amount of stone consistently.

An architect needs to consider if the quarry or natural stone supplier is capable of delivering the necessary quantities, sizes, finish and quality consistently on the whole project.

Also it is advisable to check lead times, fabrication capacity, packaging, possibility of replacing stone, documentation and other factors before approving the stone.

A Well-Written Specification Ensures Success of the Design

Specification of natural stone is essentially about forming the strong link between design intention, performance and constructibility.

The best specification defines all requirements for the project but does not exclude the unique qualities of the material from the design.

In case of architects, specification of natural stone involves going beyond the stone sample and posing a more important question:

Can this material deliver the appearance, performance and consistency the project requires?

When that question is answered before procurement begins, natural stone becomes much easier to specify, evaluate and successfully execute.