---
title: "Define crystal"
subject: "Engineering Geology I"
chapter: 3
key: "semester_3/engineering_geology_i:define-crystal"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-crystal
site: BCT Engineering
---

# Define crystal

Engineering Geology I · Chapter 3: Crystallography & Mineralogy

- Asked in: 2066 Bhadra (old course), 2 marks; 2065 Shrawan (old course), 1 marks; 2063 Baisakh (old course), 1 marks; 2062 Baisakh (old course), 1 marks (asked 4 times)
- Video: [How do crystals work? - Graham Baird](https://www.youtube.com/watch?v=PgSRAsgrKmg) — TED-Ed
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/define-crystal
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-3

## Question

Define crystal.

## Short answer

A crystal is a homogeneous solid with a regular, repeating three-dimensional internal atomic arrangement, bounded by naturally smooth plane faces meeting at fixed angles according to geometric laws. Example: quartz.

## Answer

A **crystal** is a homogeneous solid body of a chemical element or compound whose atoms, ions or molecules are arranged in a regular, repeating three-dimensional pattern (space lattice), and which is bounded by natural, smooth plane faces that follow geometric rules. Examples: quartz, calcite, halite (common salt).

Features:
- Definite internal atomic arrangement.
- Flat faces meeting at fixed angles (law of constancy of interfacial angles).
- Definite symmetry and a definite chemical composition.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Which defines a crystal best?
   A. An amorphous solid with random atoms
   B. Any rock formed by cooling magma
   C. A homogeneous solid with regular repeating atomic arrangement, bounded by natural plane faces
   D. A liquid cooled below its melting point with no faces
   Answer: C. A crystal has an internal space lattice and flat natural faces. (Key point: Definition)

2. The law of constancy of interfacial angles says:
   A. All faces of a crystal are the same size
   B. Corresponding faces of a crystal species meet at fixed angles
   C. Crystals never change shape
   D. All crystals have the same number of faces
   Answer: B. Face angles are constant for a given mineral. (Key point: Interfacial angles)

3. Which is an example of a crystal named in the answer?
   A. Glass
   B. Granite
   C. Halite (common salt)
   D. Coal
   Answer: C. Quartz, calcite and halite are crystals; glass is amorphous and granite is a rock. (Key point: Example)