The most common materials used to make gas pipes are steel, black iron, polyvinyl chloride (PVC), high-density polyethylene (HDPE) and copper. Steel pipes can be used on the ground as long as corrosion due to the environment and transport gas is not a problem. Steel and copper tubes are the most commonly used materials in the interior of buildings. Copper pipes used in gas systems must be L or K type and approved for gas.
Yellow brass pipes may be approved for indoor installations. Aluminum tubes should not be used on the ground. Aluminum is not approved in all jurisdictions. Ductile iron pipes may be approved in some jurisdictions for underground work.
When the gas line starts to leak underground, it can lead to a dead grass stain above the leak, a strong sulfur smell in the area above the leak, and a high gas bill. But natural gas is much cleaner than it was 40 years ago, and almost all fuel gas goes through a sulfur recovery unit before entering the distribution system today. The pipes upstream of the gas meter and the meter itself are usually the responsibility of the gas company. Any anxiety you may feel about your gas system can be alleviated by learning a little more about what to expect from gas system maintenance.
In my experience, both types of pipes work well, and the IRC and the National Combustible Gas Code allow both, along with stainless steel corrugated tubing (CSST) and copper tubing (with some limitations) to be used for natural gas distribution pipelines.