Step 3: Practical Gemology

Lesson 20

This is a fun test if you have rough material to spare. It’s not recommended for finished gems, as there’s a good chance the gem will break. Even with rough material, choose pieces you wouldn’t mind being ruined.

As you know, most gems receive their coloring from selective absorption, the absorption and transmission of specific wavelengths of light, which can be viewed through a spectroscope. Before testing these gems, you should confirm that they don’t have any coloring material in them. You can test this by rubbing a piece against a ceramic tile. The bit of material that rubs off, the streak, should be colorless. (For example, it does not matter how dark a sapphire or garnet is, it will always leave a colorless streak unless a coloring agent has been added to it).

Streak Testing Color Chart

Since the majority of gems leave a colorless streak, it’s easier to note which ones don’t. (Usually only opaque, metallic minerals leave a colored streak). The accompanying chart lists the ones gemologists are most likely to encounter. The only things not listed here are exceptionally rare. Any common gem…