Harder Ores alters the way ores are found, harvested, and refined into useable materials. Ores are more scarce and occur in veins (distributions controlled by Custom Ore Generation) but tend to contain much more total ore. The net result is small, scattered, mining outposts that stick around for a long time, handling one or two ore types, and which stick around for a long time before the ore vein is exhausted (particularly in SMP).
See the various sub-pages below for more information on various topics pertaining to Harder Ores.
Hard Ores integrates reasonably well with a number of mods. While not all ores are supported (especially when using two or more ore mods), any given mod should function as expected with any tech that does ore multiplication being 50% better than the tech provided by Harder Ores (3 tiny dusts instead of 2).