In this regular column, Consultant Barry Hunt answers reader enquiries. If you have a question, email it to: nss@qmj.co.uk marked for Barry\'s attention

Q: What are stone veins and are they really a problem?

The Earth is constantly shifting and existing rocks may crack in response to such movements or changes in pressure. Water or other liquids are then more able to move through the rock in these cracks and may deposit new minerals as they do so. When the cracks are completely filled, they would be considered to be veins.

Because veins essentially have the same outlines as cracks, many are mistakenly believed to be cracks and as a result are viewed with suspicion.

Thinner veins are often viewed with the greatest suspicion as these look most like cracks. However, it is actually the larger veins that may be more problematic.

Large veins are more likely to contain hidden spaces where they are not completely filled. Also the meeting of usually larger crystals growing on either face of the original fracture may have lower cohesive strength than the surrounding stone.

The most immediate problems involving veins occur when they are formed from weathered products such as clays or minerals that oxidise - for example as pyrite, often known as fool\'s gold. These can rapidly deteriorate and create additional problems such as discolouration. Some types of roofing slate suffer badly from this problem.

Most veins are innocuous as they are typically formed from the same material as the surrounding rock. Limestones and their veins often comprise calcite, with the calcite in the veins having been leached from the surrounding stone. Sandstones typically comprise silica-based minerals and are often veined by quartz, the most basic silica mineral.

However, if a sandstone were veined by calcite this could have serious consequences for the durability of the stone, the calcite being considerably softer than the surrounding silica minerals and unlikely to have an especially strong bond with them. In this case acidic decay mechanisms would tend to concentrate their efforts on the veins while the silica would remain relatively unaffected.

The various European test standards now applicable to stone must take into account the presence of veins, otherwise the potential strength and durability properties may be over-estimated. If testing has not taken veining into account there could be a risk of the values obtained overstating the strength of the stone. If these results are used to make structural calculations a failure could result.

Veins should not be a problem if a stone is properly compliant with the available standards. It would be prudent to select stone on this basis.

Barry Hunt is a Chartered Geologist and Chartered Surveyor who has spent 20 years investigating issues relating to natural stone and other construction materials. He now runs IBIS, an independent geomaterials consultancy undertaking commissions worldwide to provide consultancy, inspection and testing advice. Tel: 020 8518 8646