You need to know the type of material, ferrous, non-ferrous, alloy, etc.
It also helps to know what you are looking for. Phase balance, prior austenite grains, intermetallic phases. You can use different etchants on the same material to look for different things.
Check AWS B2.1 one of the annexes I do not remember which one.
Also check the Lincoln Electric Procedure Handbook of Arc Welding page 16.1-80 and 81 for ferrous material etching solutions.
CHGuilford
ASME IX also has a section on etchants for different types of materials.
If you are talking about carbon steel, then a very common and effective etchant for general microstructural work is NITAL. This is typically a 2% concentration of Nitric Acid in Ethanol.
Regards
Niekie