The most commonly used material for chimneys in the past couple of centuries is standard clay, which is still, today, one of the least expensive materials for this purpose. Clay flues are certainly durable, as some last for more than 75 years with regular maintenance. In fact, many clay flues from the early 20th century can still be seen poking out of rooftops in cities all over England.
Clay is hard and sturdy but it is still susceptible to cracking in the case of chimney fires. While clay flues have been used in regular fireplaces for centuries, they are not as well-equipped for gas fireplaces because of problems with combustion.
Many recently constructed chimney flues are made of metal. Typically, these are of the stainless steel variety but some consist of aluminum and copper. Although made of metal, these flues are by far the most flexible -- they are somewhat thin and constructed in the shape of a cylindrical tube, which can be shifted around, if necessary.
Because of its thinness the flue usually needs further insulation. Stainless steel flues operate in gas, wood and oil fireplaces; however, aluminum flues, because they are less sturdy, are only recommended for gas fireplaces.
A cast-in-place flue consists completely of concrete. These flues are made by placing a tube down the center of the original chimney walls, pouring wet concrete around the sides and allowing it to dry. Not only does this create a secure, even flue but also the concrete provides its own insulation. While this is a simple and usually reliable method, the main concern is that the flue may be too small to properly vent a particular fireplace, because once the cast is in place, it is a permanent fixture.
Multi-flues consist of more than one flue traveling up the same chimney. These are fairly common and are often recommended in houses with more than one fireplace to keep the smoke and fumes from each separate. Multi-flues can come in any material, including metal, clay or concrete, each of which has its own benefits and drawbacks.