Chelsea & Hanneman Filter Reactions — Reference Chart

Common Chelsea filter lookups: Emerald Chelsea variable  ·  Ruby Chelsea red  ·  Alexandrite Chelsea red  ·  Chrome Diopside Chelsea red  ·  Tsavorite Garnet Chelsea red  ·  Demantoid Garnet Chelsea red  ·  Aquamarine Chelsea inert

The Chelsea filter (also called an emerald filter) transmits only deep red and yellow-green light. Chromium-colored stones transmit the deep red and appear pink to red through the filter, while most iron-colored look-alikes absorb it and stay green or grey. It was developed for separating emerald from its simulants — though the modern reading is more nuanced: emerald's Chelsea response is recorded here as variable, because iron content varies by deposit and many natural emeralds do not redden, while some synthetics glow vividly. A red flash is a chromium indicator, never an identity by itself.

The Hanneman filter set (developed by Dr. W. William Hanneman) extends the same principle with differently tuned bandpasses — including aquamarine and ruby filters designed to be used in pairs with the Chelsea. The Hanneman column below records the engine's expected response where one is defined; species with no recorded response show an em-dash. Published Hanneman data is scarce, which is exactly why the column is here.

Filter tests take seconds, need no contact, and work on parcels — sweep a tray of green stones under the filter and the chromium-bearing candidates announce themselves. Follow up any reaction with the UV lamp and the refractometer: the trio reads the same chromium chemistry three different ways.

Which gemstones turn red under the Chelsea filter?
10 species in this chart are recorded red through the Chelsea filter: Alexandrite, Chrome Diopside, Chrome Tourmaline, Demantoid Garnet, Enstatite, Ruby, Star Ruby, Synthetic Color-Change Corundum, Synthetic Spinel, Tsavorite Garnet. All are chromium-bearing. One species — Paraiba Tourmaline — is recorded green.

What is a Hanneman filter?
A set of color filters developed by Dr. W. William Hanneman that extend the Chelsea principle with different transmission bands, used for separations such as aquamarine from blue topaz and synthetic from natural emerald. This chart records a defined Hanneman response for 103 of the 130 species, including 9 that read red.

Does emerald always look red under a Chelsea filter?
No — emerald's response is recorded as variable in this chart. Iron in the stone quenches the red transmission, so many natural emeralds (notably iron-rich deposits) stay green under the filter while chromium-rich stones and many synthetics show pink to red. Treat the filter as a screening indicator, not an identity test.

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See also: Full Reference Database  ·  How to Use a Chelsea Filter  ·  Exam Study Guides