Minerals are:
1. Naturally occurring
2. Inorganic
3. Solids
4. Minerals have a definite chemical composition
5. Minerals have an orderly internal crystal structure

Minerals are the building blocks of rocks. Each mineral has different physical and chemical properties which allows it to be identified. Although mineral occurs in different varieties but we would discuss main types & their impression on rocks.

Silicate class of minerals
This is the most important, largest, most complicated & most interesting class of minerals. Basic unit of silicates is SiO4, tetrahedron shaped anionic(-ve charged) group with a negative four(-4) charge.
Single –ve charge on each oxygen ion of this basic unit (SiO4) combines with +ve charge of Si(It is having Four positive charge) of another basic unit (SiO4).In this way they are joined & bounded together to form a solid appearance.
Silicates are divided into many classes on the basis of structure(Note here that not by vertue of chemistry).

Carbonate class of minerals
These are those minerals which are having carbonate ions.Typically carbonates are Transperant,Light coloured,Good cleavage & give whitish streak to the rock in which they are present.Acid test is best test for carbonate presence in any rock.Because calcite always will effervesce (bubble) when even cold weak acids are placed on specimens.
 

Quartz: -

Quartz (Sio2), silicon di oxide is the common mineral in most types of stones. This mineral is part of tectosilicate subclass of Silicate family of minerals. Tiny pieces of quartz are known as sand. The quartz has hardness of 7 MOH (Measurement of hardness scale). Quartz is generally used in producing optical instruments and electrical devices. Impurities, in the rocks at the time of formation, causes the quartz crystal to have different colors like milky white, yellow, smoky or purple. 

Feldspar: -It is one of the most abundant mineral in rocks, which are near to earth’s surface. “‘Calamite” is the; best quality feldspar clays used by potters. Unlike quartz when feldspar is exposed to atmosphere they break easily. All feldspar contains silica and aluminum. feldspar can possess glassy white, blue, green or red color. Feldspar is at number 6 on MOH scale.

Magnetite: - It is the best source of iron in the world. This ore has rustic or red color because when Reaction between iron in this ore with water and oxygen, turns the iron into oxide (rust), which is red dust in color. This is the mineral, which is naturally magnetic in nature. Its hardness on MOH scale is 5 to 6.  

Pyrite: - This is another iron ore which is formed by reaction of Sulpher & Iron. That is why it is known as iron sulfide also. Its crystal has golden color. It’s hardness is 6.5 on MOH scale. 

Graphite (carbon): -Graphite is pure carbon just like diamond. But it has hardness of 1 to 2 on MOH as compared to diamond, which is placed at the highest place i.e. 10 on MOH. Graphite actually turns into diamond at high temp & pressure in earth’s surface. Graphite gives black or darkish color to any rock. 

Calcite (caco3): - It is calcium carbonate.Caco3 is found in limestone and marble (which is resulted from metamorphism of lime stone). Calcite acts as cementing agent that binds the sediments of rocks together. Above lime also states that calcite is main constituent of sedimentary rocks. Its crystals are shaped in perfect Rhomboids(hexagon). Calcite has hardness of 2 on MOH scale. 

Dolomite: Dlomite is calcium-magnesium carbonate. It is also used as source of magnesium. It is very similar to calcite & gives pinkish or pinkish white colour to any stone. On MOH its hardness is 4. 

Micas:

These are minerals of Phyllosilicate subgroup of silicate mineral group, composed of varying amount of potassium, magnesium, iron, aluminium, silicon & water. It is used to manufacture electrical devices. Mica can be split in vary thin layered sheets & it is so soft that we can scratch it with our nails. Best mica is mined near Moscow (Russia) that is why pure, light colored, clear mica is known as Muscovite (Potassium Aluminium Silicate Hydroxide Floride ). Biotite (Potassium Iron Magnesium Aluminium Silicate Hydroxide Floride) is another type of mica, which possesses dark, black or brown color because of iron and magnesium impurities. Biotite is a common mineral in siliceous (a rock rich in quartz) intrusive rocks such as granite. It is less common in siliceous extrusive rocks.  

Galena: -It is source of lead on earth i.e. galena is lead ore. Its chemical symbol is PbS. Galena can also contain silver. Previously galena was used for making pencil lead. But now graphite is used as pencil lead instead of galena because galena is poisonous for human. It is slightly harder than graphite and has metallic luster and black to dark gray in color. 

Hornblende: - This mineral contains magnesium, iron, silica and aluminum. It occurs in many igneous rocks. Hornblende gives green to dark color to a rock. Hornblende Ca2(Mg,Fe,Al)5(OH)2[(Si,Al)4O11]2. Hornblende is common in many intrusive igneous rocks ranging from granite to gabbro.It is also present in medium grade metamorphosed rocks. Hornblende occurs in some volcanic rocks, but it is less abundant in extrusive rocks than in intrusive rocks. 

Kaolinite: -This is a clay material which is most abundant mineral on earth. It is very soft (2 to 2.5 on MOH) & possess whitish and pinkish color characteristics. It is used in ceramic industry to make clay. Its chemical composition is Al2si2o5(OH). 

Pyroxene: - This mineral is main constituent of basalt. Pyroxenes are silicates of magnesium, calcium and iron. Augite is a common pyroxene, which is very abundant in igneous rocks (basalt etc). 

Zeolites: -

The zeolite group of minerals are hydrous Ca-Na aluminum silicates. Only four of the group are listed here; there are many more varieties, and many look similar in thin section. The zeolites are listed here as secondary minerals in igneous rocks, a common environment for them, but they are widespread in some marine and fresh water deposits as well.  

Basic concepts of Colors in Rocks

Only a little bit of iron (even less than 1%) may be enough to make the entire rock look red.

Here are some generalizations about the color of rocks:

• Rocks may be various colors, depending on what they are composed of and whether they were formed under oxidizing conditions.
• Iron minerals in rocks deposited in deep water, such as in the ocean or deep lakes, are less oxidized, and these rocks tend to be black or gray.
• If rocks are deposited in shallow water, they may be more greenish or brown.
• Rocks deposited on land instead of under water are more oxidized, especially if they were deposited in wet environments, and so tend to be reddish or tan.
• White rocks can be formed in sand dunes, beaches, lakes, and shallow sea.
• Rocks formed at high temperatures, such as from the solidification of molten rock, or at great depths tend not to be red because the iron minerals cannot oxidize under these conditions.
• If rocks sit at the surface under wet conditions, the iron minerals can be oxidized, turning the rock red.