Suppliers to the Jewellery Trade

Technical Information Sheet
The Chelsea Colour Filter
Simple verification of coloured gems

A. For assistance in distinguishing Emeralds.

The Chelsea Colour Filter, also known as an Emerald Filter, was originally designed to distinguish genuine emeralds from emerald imitations and simulants.

Emeralds transmit deep red light but absorb light in the yellow-green area on the spectrum. The Chelsea Colour Filter is the equivalent of two colour filters which have been carefully selected to transmit only deep red light and yellow-green light.

When green stones are strongly illuminated and viewed through the filter, most emeralds appear distinctly yellowish pink to red (depending on the stone's depth of colour). However, imitation emeralds (e.g., green glass, soude emeralds, green garnet-topped doublets and most green tourmalines) appear a dark green when illuminated.

An emerald's pink/red appearance when viewed through the Chelsea Colour Filter is due to its colouring element, chromium, which is absent from nearly all emerald look-alikes (the main exceptions being green demantoid garnet and some chrome green tourmalines, all of which may also appear pink through the filter). It should also be noted that some genuine emeralds (particularly those from South Africa) may fail to exhibit pink through the filter because their chromium colour characteristic is "inhibited" by the presence of iron oxides.

When synthetic emeralds first appeared on the market, the Chelsea Filter became less reliable as a check for natural emeralds because the synthetic stones also contained chromium and appeared red through the filter. However, a trained eye will observe that these synthetics demonstrate a much stronger red through the filter than the natural stone because of excessive chromium. In addition to emeralds there are other gems, such as ruby and red spinel, which owe their colour to chromium and also appear red through the filter (the synthetics of these stones usually producing a brighter red that the natural stone).

B.  For detecting synthetic gems coloured by cobalt.

The Chelsea Colour Filter can also be used in detecting synthetic gems coloured by cobalt because these also appear pink through the filter. (Apart from a rare natural blue spinel and very rare cobalti-calcite, no natural gemstone contains cobalt).

As a detector of cobalt in synthetic gems, the Chelsea Colour Filter can distinguish aquamarine, sapphire and blue zircon from synthetic blue spinel, blue cobalt glass and synthetic blue cobalt quartz (which appear pink).

Synthetic dark green spinel (also coloured by cobalt) appears pink through the filter, which differentiates it from the majority of green tourmalines.

1. Select a strong white light source (preferably a filament lamp).
2. Position the gemstone under the light.
3. Hold the filter close to the eye (never at arm's length!) and inspect the gem for signs of pink or red colour, while noting the intensity of the colour. Check the results against the preceding notes.

WARNING: Stones should never be left more than a minute or so, as some may be damaged by the heat of the focused beam from a high-intensity lamp.

Note: Test results obtained using the Chelsea Colour Filter should not be relied upon solely as positive identification of a gemstone, but should be confirmed by other evaluative procedures.