material=* – describes the main material of a physical feature.
It is most commonly used to describe the material of monuments, memorials, statues, walls, fences, but can be used for any other physical object except those listed below, which use other tags:
Tufa and/or travertine - a soft sedimentary rock formed by the chemical precipitation of calcium carbonate minerals from fresh water, typically in springs, rivers, and lakes, especially in karst around waterfalls
Metal grid (depending on the model also called wire mesh or wire grid, comparable is also perforated metal) is a specific type of material=metal. It is often used for seats and backrests of benches, sometimes also for outdoor tables.
Weathering steel, sometimes called COR-TEN steel, are steel alloys that obtain a distinctive stable rust-like exterior when exposed to the weather. Typically used as building cladding and in abstract sculptures.
But, if someone wants to provide a detailed info for a surface, material=* can be used as an extra detail. This is friendly both for data consumers looking for extremely detailed info and for more general-purpose ones. For surface=paving_stones there is paving_stones:material=*, which can also be used.
For example, the distinction between surface=unhewn_cobblestone and surface=paving_stones is very important for bicycle routing, while distinguishing between marble surface and granite surface and concrete surface is of lesser importance.
surface=metalmaterial=steel is preferable over surface=steel as it allows more general-purpose data consumers to process data without failing as soon as someone will add even more detailed info about surface.