Chelsea Filter Test
Overview
The Chelsea filter is a simple color filter that helps separate emeralds from their simulants and can aid in identifying certain other gems. It transmits only deep red and yellow-green light.
How It Works
Chrome-bearing gems (like natural emerald) absorb yellow-green and transmit red light through the filter, appearing pink to red. Non-chrome green gems remain green or grayish.
Equipment Needed
- Chelsea filter
- Strong incandescent or LED light
- White background
- Known reference stones
Procedure
- View the gem in good incandescent light without filter
- Hold the Chelsea filter close to your eye
- Observe the gem through the filter
- Note the color seen (red, pink, green, or gray)
- Compare with known reference stones
Typical Values / Results
| Gemstone | Value / Observation |
|---|---|
| Colombian Emerald | Pink to red (chrome) |
| Zambian Emerald | Often weaker reaction |
| Green Glass | Remains green or gray |
| Chrome Tourmaline | Red (similar to emerald) |
| Synthetic Emerald | Strong red (high chrome) |
Limitations
Not definitive—chrome tourmaline and some synthetic emeralds show similar reactions. Some natural emeralds show weak or no reaction. Should be used with other tests.