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Natural materials, such as stone, are the most difficult building elements to specify and procure. The fuss at the British Museum testifies to that. The pre-contract procurement strategy, tender-evaluation process and post-contract award procedures are all critical and require co-operation between client, designer, contractor, supplier and installer. Unfortunately, this is not always possible – the result of the seemingly endless contract variations in use today.

The simplest scenario is for the designer to select the stone it desires pre-tender, reserve sufficient quantities at the quarry, fix the price and then nominate the supplier in the specification. This method, however, presupposes the client's willingness to take the risk of pre-ordering, the contractor's ability to find an installer who will be able to control the supply on the terms agreed and the designer's willingness to make a final decision as to the stone it wants. But regardless of these difficulties, this is the only way to guarantee that the designer gets the right material.

At the opposite end of the spectrum are projects funded in whole or part by the public purse. Such projects have to comply with European regulations, and that introduces complications. Most importantly, the designer is unable to specify a preferred product, as this breaches the requirement for advertisement and open competition within the European Union. This makes specifying natural materials difficult, because they are often selected on the subjective criteria of colour and appearance. Other materials may comply with technical or performance criteria or be of a similar type, but look completely different – making them inappropriate.

So how can specifications be written in circumstances where competitive tenders must be sought and EU rules complied with? It is important to remember that the rules of procurement apply to tendering only to set a level playing field for all EU members. The objective, therefore, should be to produce a contract specification that confirms precisely what has been agreed following tender submission, how much the goods will cost and how their quality will be monitored. The contract specification should confirm the type and source of the materials to be supplied, and that sufficient material is available in the time required. That said, how do you get from a fair descriptive tender specification to a firm prescriptive contract specification, and then how do you ensure compliance? During the procurement phase, the key is clear performance and design intent requirements linked to the tenderers' provision of samples. This must be followed up with quality assurance methods during the construction phase.

Other materials may comply with technical or performance criteria but look completely different, making them inappropriate

Using stone as an example, the tender specification must clearly state: the design criteria, including scope; description of visual intent; performance requirements of the material and structure to which it will be fixed; prevailing environmental conditions; type, accessories, tolerances and finishes; workmanship requirements; and, most crucially, the number and size of samples, and the testing regimes that will be used. It is important to remember that specifying a type of natural material can produce numerous "compliant" sources, which must be narrowed down to those that are acceptable in the pre-contract stage.

Reviewing, accepting and rejecting samples provided with the tender is the key to the success of the procedure. This should never be left until after the contract award. Agree a range of acceptable colours, textures and limitation of imperfections before awarding the contract and keep them safe. Include the material source and supplier by name in the contract specification and confirm the existence of the agreed samples. This results in a contract in which it is clear what is being supplied.