Rock minerals help restore a broad spectrum of minerals to the soil. Rock minerals are best applied in a powder or similar form called rock dust which is generally obtained from rock crushing or cracking processes (thus the names crusher dust and cracker dust). The source rock material will determine what minerals are supplied by the rock dust. Rock mineral helps to reduce compaction and improve water-holding capacity. It also provides an environment that supports the proliferation of soil microbes. As the increased soil life slowly accesses the beneficial minerals, plants also benefit. The major minerals available from different rock dusts are potassium (K), calcium (C), phosphorus (P), and magnesium (Mg). Rock dusts also provide trace elements such as silicon (Si), iron (Fe), and zinc (Zn) to name a few. Nitrogen will generally not be obtained from rock minerals.
Types of Rock Dusts
Igneous Metamorphic Rock
Basalt, Greenstone, or Hornfels rock dusts are types of igneous metamorphic rock. They provide calcium, magnesium, and phosphorous to the soil. They also provide many other valuable trace elements such as iron and silicon. Note that Greenstone generally provides much higher levels of phosphorous than Basalt or Hornfels rock.
Crushed granite and naturally decomposed granite (aka deco) provides potassium, calcium, and magnesium. They also provide many other valuable trace elements such as silicon and iron. Reference the Granite wikipedia entry for a worldwide average of the chemical composition of granite. Note that deco often contains fine clay particles, so it is best worked into the soil (really as with all rock minerals).
Not too fine
While fine particles of rock minerals are best for the soil life as they can be broken down faster, it can make it hard to remember where and how much you have used. Thus rock dust with some very small pebbles through out it can make things much easier for the gardener. This is because the pebbles remain intact long after being applied to the soil.
How much to use
It really depends on your soil type as to how much you should use. Ideally a soil analysis of your soil and the rock dusts should be done. However the reality is that this can be expensive, thus most people do not gown down this path. However it you have a granite based sub-soil such as deco, then you will be unlikely to need any of this type of rock dust when preparing your soil. The only exception might be if a plants roots will not penetrate deep enough to mine the sub-soil (such as is likely to be the case for veggie gardens or trees with shallow root systems such as citrus).
When you are using rock minerals for the first time, you want to be generous with it. As the rock minerals break down slowly, what you are trying to do is create a reserve of the beneficial elements for the soil life to access. Thus in turn will benefit your plants.
Another thing to consider when applying the rock minerals is what type of plant it is for. In the case of trees where the soil is prepared once, it is best to use more in the planting hole than you might use in a garden with short lived annuals. This is because you have one chance to get the soil as well enriched as possible for a long-lived tree.
As a general guide, you should be able to see the larger particles of the rock minerals through out the soil when the soil is less than a couple of metres away. However from a distance the rock minerals should not be visible. If they are visible, then you might have used too much which might cause your soil to have reduced water holding capacity. This can be corrected over time by working more organic matter into your soil.