Gabbro
It is a coarse-grained mafic igneous rock which consists of plagioclase feldspar and pyroxene. Olivine, sometimes also forms a major constituent. Other minor one are hornblende, biotite, quartz, magnetite and ilmenite.

Gamma rays
It is form of electromagnetic radiation containing the shortest wavelength of the spectrum. These rays can easily penetrate matter.

Gangue
It is the undesirable or unwanted minerals in an ore deposit.

Geology
It is the science and the study of physical structure of the earth along with the areas of rocks and minerals.

Geologist
It is a term for an individual who is practicing in the subject of geology.

Glassy (luster)
It is the synonym of vitreous luster.

Gneiss
It is a banded or layered metamorphic rock, which is similar to the composition of granite.

Glauber's salt
It is a salt made of sodium sulfate (Na₂SO₄.10H₂O) and it is used in the manufacturing of paper and glass and in stomach medications. It is extracted from Glauberite mineral.

Grain
It is a term used for individual component belonging to a grainy aggregate.

Gravel
It is the accumulation of tiny, smooth, rounded rock that is deposited by a stream or river.

Gravity
It is the force which attracts any matter to the center of the earth. Sometimes, specific gravity is also shortened to gravity.

Greasy (luster)
It is the luster of a mineral which appears coated with grease. Some minerals are even coated with chemicals to induce a greasy luster.

Group
It is an aggregate of crystals.