Dusts are gained from Macerating their respective ores. With the exception of Coal, which only returns one dust per piece, all ores when macerated will give 2 dusts of their type. Ingots can be macerated down again to form one dust.
From Ore Blocks
Dusts, apart from Coal Dust and Clay Dust, can be smelted into ingots, allowing for 2 ingots per ore. Dusts may also be used in a variety of recipes for machines and items.
Small Pile of Iron Dust
You can harvest Small Piles of Iron Dust from a crop called Ferru, from the Industrial Craft mod, two of which can be crafted into iron dust. You can also craft them from tin and copper dusts.
However, note that tin has the same EMC value of iron. You can convert Tin to Iron at a 1:1 ratio using Equivalent Exchange. This is much more effective than creating "Small Piles of Iron Dust".
Note that Clay Dust is different from Clay.
Piles of clay as extracted from the ground can not be macerated. Four piles of clay can be crafted into a block of clay which can then be macerated into two Clay Dust.
Clay dust is used to make construction foam which can then be compressed into CF pellets to be used in a CF Sprayer or a CFPack.
Nether ores can, like any other ore, be macerated. Unlike when smelted, you receive two ore blocks instead of one - effectively quadrupling your mining yield from the nether.
While highly profitable, Gold and Iron Nether ore is bugged in SMP and refuses to be macerated properly. Instead, it displays as macerated until the player tries to remove it, whereupon it returns to the input slot and turns back into ore.
Nether Redstone ore can be macerated for a guaranteed drop of six units of Redstone dust, the same as if you smelt it in a furnace.
Coal to Diamond
Coal Dust can be turned into Coal Balls like this.
From there, you can then use a Compressor, to compress the Coal Balls into Compressed Coal Balls
After that, you can then turn 8 Compressed Coal Balls and 1 Obsidian, Block of Bricks or Iron Block into a Coal Chunk
Just compress the coal chunk, an the result is a diamond.
Effectively turning 64 Coal, 8 Flint, and 1 Obsidian/Block of Bricks/Iron Block into a Diamond
Note that if you have an Energy Condenserit would be much more effective to turn 64 coal into a single diamond. This would use less resources (not counting any machines used in the process)