Limestone is rated according to how hardwearing it is by ASTM International, an organization that rates various materials according to certain performance criteria. Low-grade limestone, type I, may not be hardwearing or water-resistant enough if you live in a wetter part of the country or if you're planning to use it for a high-traffic footpath. Type III limestone, on the ASTM scale, on the other hand will stand up to almost any outdoor use or weather conditions.
You can use limestone to build garden walls; after all, it was used in the building of the Empire State Building in New York! Limestone actually comes from sedimentary rock, and it contains a mixture of plants and fossils, as well as mud, sand and rock; it's all compacted together and comes in a variety of colors from white to black. It even comes in red, so you can use a mixture of colors to create interesting patterns when you use it for your paths and patios.
You can buy limestone flagstones for footpaths, which are one to two inches thick (2.5 to 5.1 centimeters). If you want a more formal footpath, choose square or rectangular slabs, called "fully-dressed" stone in the trade. For a more informal walking area use irregular cut limestone, otherwise known as "semi-dressed" stone.
By using a thin veneer of limestone on an otherwise regular wall you can create a full stonewall effect without having to use too much stone. Just remember to use the right grade of limestone when creating your hardscape design for the best results.