why does some rocks and pebbles have colours?

Answer 1

Different mineral compositions lead to different colors. Rocks with high concentrations of quartz will also sparkle a bit more. Different formations also have an effect (different types of rock-sedimentary, igneous and metamorphic).

I have yet to see invisible rocks though.

Answer 2

All rocks and pebbles have colours - it is just a matter of what colour. The main thing it comes down to however is the minerals present and the oxidation states of the elements within the minerals.
As already mentioned, iron gives a red/orange colouring to some minerals, however this only happens under conditions when the iron atoms in the mineral crystals have been oxidised from a +2 state to a +3 state. Conditions under which this takes place are usually dry and arid, so generally when you see something like a red sandstone (for example the Devonian sandstones in southern england), these date back to a time when the particles forming the sediment to make the rock were eroded from a desert environment.
Dark colourings such as black, dark brown and greeny grey are usually associated with mafic vocanic rocks which are high in elements such as iron and magnesium and originate at mid ocean ridges or hot spots. Examples of these types of rocks are basalts, gabbros etc and can be found in places like Hawaii.
Paler igneous rocks such as granites, pumice and rhyolites appear this way as the same magma which formed the darker igneous rocks, rather than being erupted, underwent a long process of fractionation. This filters out the more dense minerals, which contain the elements like iron, leading to the majority of the erupted lava being pale silica (quartz), with other lighter coloured minerals such as feldspars (comprising of elements such as aluminium and potassium) predominating. This leads to rocks which are pale grey, pale brown and sometimes pinky in colour if orthoclase feldspar is present.
Green coloured rocks and pebbles can be found in a number of forms however the cause is usually unoxidsed iron in the +2 oxidation state in the minerals present. Ultramafic rocks (from the top of the mantle/bottom of the crust which are usually only found on land at times when the oceanic crust has been uplifted e.g. the Troodos ophiolite in Cyprus) have a very high iron content in minerals like olivine and peridotite making them commonly dark green however chlorite also plays a large role in the green colouring of many rocks. The mineral glauconite sometimes occurs in sediments such as the "greensand" layer in southern britain, and is again due to iron, but the rock formed under depositional conditions.
Metamorphic rocks may also be green if the original rock, temperature and pressure conditions place them in the "Greenschist" section (facies) or metamorphism, and "Blueschist" may form if the metamorphic conditions are low heat/high pressure, though this rarely occurs in nature.
Finally, the rock/mineral obsidian is black as the lava which formed it cooled too rapidly for the atoms to arrange themselves into crystals, meaning that all wavelengths of light are absorbed.

I could go on... but I think that is a good start and you may want to wikipedia a couple of the things i have said!

Oh and when rocks seem to shimmer and shine, that is more commonly due to the presence of the flaky mineral muscovite mica which is associated with early metamorphism.

Answer 3

depends on what elements they have in them. for example, rocks with iron in them tend to have a red or brown in them. rocks with calcium tend to have white depisits in them. cobalt will have a blue tint or sometimes green