Which one of the following rocks is not classified as an igneous rock in geology?

Introduction / Context:
Rocks on Earth are broadly grouped into three major classes: igneous, sedimentary, and metamorphic. Competitive examinations often test basic examples of each category. This question asks you to identify which listed rock is not igneous. Recognising common igneous rocks such as granite, basalt, and gabbro, and contrasting them with sedimentary rocks like dolomite, is an important part of general geology and geography knowledge.

Given Data / Assumptions:
- Four named rocks are provided as options: granite, basalt, dolomite, and gabbro.
- Only one of these rocks is not formed from cooling and solidification of molten magma or lava.
- Standard geological classification is assumed, without considering rare or unusual rock varieties.

Concept / Approach:
Igneous rocks form from the cooling and crystallisation of molten material. When this cooling takes place beneath the surface, intrusive igneous rocks such as granite and gabbro develop. When it occurs at the surface from flowing lava, extrusive igneous rocks such as basalt form. Sedimentary rocks, in contrast, are formed by deposition and lithification of sediments, chemical precipitation, or biological processes. Dolomite is a sedimentary rock rich in the mineral dolomite, often formed by alteration of limestone or direct chemical precipitation. Knowing typical examples of each class allows us to distinguish the odd one out in such questions.

Step-by-Step Solution:
Step 1: Recall examples of intrusive igneous rocks. Granite and gabbro are both classic intrusive igneous rocks that crystallise slowly below the Earth surface. Step 2: Recall examples of extrusive igneous rocks. Basalt is a common extrusive igneous rock formed from lava flows at or near the surface. Step 3: Identify dolomite. Dolomite as a rock, often called dolostone, is primarily a sedimentary rock composed of the mineral dolomite, usually formed by alteration of limestone or chemical precipitation in marine environments. Step 4: Conclude that granite, basalt, and gabbro are all igneous, whereas dolomite belongs to the sedimentary class, so dolomite is the rock that is not igneous.

Verification / Alternative check:
Textbooks on physical geology and introductory Earth science consistently list granite, basalt, and gabbro among major igneous rocks. They describe granite as coarse grained and rich in quartz and feldspars, basalt as fine grained and mafic, and gabbro as a coarse grained mafic intrusive equivalent of basalt. In contrast, dolomite is grouped with sedimentary carbonate rocks and is often discussed together with limestone in chapters on sedimentary environments. Checking any standard classification table confirms that dolomite is not an igneous rock, which verifies the conclusion drawn from conceptual reasoning.

Why Other Options Are Wrong:
Granite: Clearly identified as an intrusive igneous rock, so it is not the correct choice for a non igneous example.
Basalt: A typical extrusive igneous rock found in oceanic crust and lava plateaus, so it is not the odd one out.
Gabbro: Another intrusive igneous rock, coarse grained and mafic, and therefore still igneous and incorrect for this question.

Common Pitfalls:
Learners sometimes confuse rock names with mineral names or with commercial terms used in the building industry. Dolomite as a mineral and a rock can be mistaken for a type of igneous rock because it is less widely discussed than granite or basalt in school textbooks. Another pitfall is to assume that any rock used as a building stone could be igneous. To avoid these mistakes it is useful to memorise at least a few standard examples of each rock class: granite, basalt, gabbro as igneous; sandstone, shale, limestone, dolomite as sedimentary; and marble, slate, gneiss as metamorphic.

Final Answer:
The rock that is not classified as igneous is Dolomite, because dolomite is a sedimentary carbonate rock rather than an igneous rock formed from solidified magma or lava.