Hardness - is geologically caused by calcium and magnesium carbonates from dissolved limestone in groundwater.  It's indicated by white or gray crusty scaly build-up on glass surfaces, failed water heaters, dingy laundry and soap scum problems.  The water treatment industry considers that treatment may be recommended when levels exceed 3.5 gpg. (grains per gallon) or when specific types and concentration of iron and manganese are present.