Why the test works
A crystal lattice may contain directions where atomic bonds are weaker. Breakage along those directions produces cleavage planes with repeatable geometry.
Minerals without useful cleavage may break by conchoidal, uneven, splintery, or earthy fracture. A sample can display both cleavage and fracture on different surfaces.
How to perform the test
- 1
Observe before breaking
Rotate the specimen under a single light. Search existing chips for flat reflections that flash together.
- 2
Find repetition
A single plane may be a crystal face or accidental break. Cleavage is supported when several parallel surfaces recur through the same mineral.
- 3
Count directions
Group parallel planes as one direction. Then look for a second or third nonparallel set.
- 4
Estimate angles
Note whether directions meet near 90 degrees or at distinctly oblique angles. Do not demand perfect geometry from a rough hand sample.
- 5
Record fracture too
Describe curved shell-like, uneven, splintery, or earthy surfaces where cleavage is absent or interrupted.
How to read the result
One excellent direction; thin elastic or flexible sheets
Characteristic of mica-group minerals, though sheet behavior and color still matter.
Two directions near 90 degrees
Common in feldspars and pyroxenes. Striations, angle precision, color, and hardness help separate them.
Two directions near 60 and 120 degrees
Supports amphibole-group minerals such as hornblende.
Three directions at 90 degrees
Cubic cleavage, seen in minerals such as halite and galena.
Three directions not at 90 degrees
Rhombohedral cleavage, typical of calcite and dolomite.
Smooth curved, shell-like surfaces
Conchoidal fracture supports quartz or glass, but does not distinguish natural material from manufactured glass by itself.
Examples from the LithoMap guide library
Common mistakes
- Calling a crystal growth face cleavage
- Counting each parallel surface as a separate direction
- Judging the angle from one distorted chip
- Assuming every shiny plane is cleavage
- Breaking a specimen before inspecting surfaces that already exist
Test safely
- Prefer existing broken surfaces. Do not strike a specimen unless you own it, have permission, and can control flying chips.
- Wear eye protection and keep other people clear of the work area.
- Do not break fibrous, dusty, sulfide-rich, lead-bearing, radioactive, or culturally significant material.
- Breakage is destructive and cannot prove a mineral identity on its own.
This method supports preliminary field identification. It is not laboratory confirmation, appraisal, hazard clearance, proof of origin, or permission to collect or alter a specimen.
Frequently asked questions
Is a flat crystal face the same as cleavage?
No. A crystal face formed during growth; cleavage formed during breakage. Cleavage is identified by repeated parallel surfaces within broken material, not by flatness alone.
Can a mineral have both cleavage and fracture?
Yes. A break may follow cleavage in one area and cross the structure irregularly elsewhere, especially in small, intergrown, or weathered grains.
Why are cleavage angles approximate in hand samples?
Surfaces may be incomplete, stepped, altered, or partly hidden by other minerals. Use angle categories together with hardness, habit, and other properties.
Authoritative sources
LithoMap wrote this guide in original language using the physical-property standards and safety information supported by these sources.
- Minerals and their physical properties U.S. National Park Service
- Geology Lesson 2: Global Building Blocks U.S. National Park Service
- Kansas Rocks and Minerals: Mineral Identification Kansas Geological Survey
Editorial review: September 18, 2026.