Activity

Mineral testing: hardness and streak colour

Summary
Try some simple tests to help identify minerals: hardness relative to a steel knife blade and streak colour.
Science content
Earth/Space: Rock cycle, Earth Materials, Natural resources (5)
Lessons activity is in
Materials
  • minerals to test, ranging in hardness/streak colour e.g. gypsum (2/white), calcite (3/white), bornite (3/black), copper (3/grey or pink), pyrite (6/black), hematite (6/brown), quartz (7/white or none)
  • stainless steel butter knife
  • unglazed porcelein e.g. scraps from a You-paint pottery studio or tiles with unglazed backing
Procedure

Rocks may be made up of one mineral, or a group of minerals whose crystals have grown together. To help identify a rock by the minerals in it, there are a variety of tests including test for hardness and streak colour.
1. Testing for hardness:
The Moh’s scale is a scale of relative minerals hardness from 1 (talc; very soft) to 10 (diamond; very hard). It is used to help categorise and identify minerals. The hardness of a mineral can be measured by how easily it is scratched, either by other minerals of known hardness or with calibrated tools.
Students use a steel butter knife blade (scratched by minerals more than 5 or 6) to test the hardness of minerals to help identify them (see attached example). Other common materials to test for hardness are: fingernail (2.5), copper penny (3-3.5), steel nail (5) , pocket knife (5.5), window glass (6), steel file (6.5).
2. Testing for streak colour:
The overall colour of a mineral may vary if it contains impurities or has an irregular crystal structure, whereas the powder of a mineral has a more consistent colour. Scraping a mineral across a “streak plate” (unglazed porcelain) produces a line of powdered mineral whose colour can help identify the mineral. (If the mineral is harder than the streak plate (~7 on Mohs’ scale) another crushing method must be used.)
Scrape the minerals across the porcelain to examine their streak colours and identify them (see attached example).

Notes

These tests can be used to investigate locally mined minerals and their products e.g. bornite is mined in BC for its copper content (native copper sources are rarer). Much copper is recycled.

I found that native copper had a grey streak, though it is documented as copper red colour. Therefore I suspect that the "copper" I bought has traces of other minerals - iron?

Grades taught
Gr 1
Gr 2
Gr 3
Gr 4
Teaching site
Bayview Elementary Science Club
Gordon Elementary Science Club