While there are two types of pipes that can be buried legally for gas line use, they are not equally popular or economical. Corrugated stainless steel tubing, or CSST, can be encased in conduit and used underground, but due to the high price of stainless steel products and brass fittings, this is not always an economical option. The more cost-effective solution would be polyethylene tubing, which is yellow in color and should be clearly identified for natural or propane gas usage.