05.02 Colour Filters

In this lesson, you will understand more about colour filters, with a special focus on the Chelsea colour filter.

Invented 80 years ago in Chelsea, London, this filter was initially created to help differentiate emeralds from their numerous imitations on the market, effectively absorbing most wavelengths of light while allowing specific red and green hues to pass through. While the Chelsea colour filter can make a natural emerald appear bright red when viewed through it, its effectiveness has decreased over the years, especially with the rise of advanced synthetic gemstones. You’ll discover that the filter is not reliable for distinguishing between natural and synthetic emeralds today, as even some new emeralds don’t react as expected. This lesson also touches on other materials like blue sapphires and garnets, further emphasizing the limitations of colour filters in gemstone identification. So, while colour filters can still have their uses, particularly with certain green stones, they shouldn’t be relied upon for critical judgments in gem identification.