Principle
A colour filter alters the spectrum reaching the observer. The apparent colour depends on the lamp, gem absorption and viewing conditions.
Practical observation
Use suitable white illumination, hold the filter close to the eye and view light transmitted through a transparent stone or reflected by an opaque specimen.
Limits
A red or green response does not uniquely identify a gemstone, its geographic origin or whether it is natural. Confirm with additional tests.
Illustrated guide
Frequently asked questions
Can the Chelsea filter prove that a stone is emerald?
No. It provides a screening response only; more tests are needed for identification.