Autumn 2020 Gemmology Bulletin Phenomenal Glass Gemstones

By James Evans, FGA

At once the most undervalued of gemstones, and the most remarkable…

Glass is unrivalled in the range of optical phenomena00 it displays.

Goldstone Cat’s-eye Glass (aventurescence) (chatoyancy) This glittering gemstone, also This gemstone displays a- known as ‘Aventurine Glass’, bright and even cat’s-eye. is the origin of the term A more fibrous, shimmering aventurescence. The term was chatoyancy can be seen within later applied to varieties of quartz ‘Victoria Stone’, though the skills and feldspar that contain glittering flakes of mica. required to produce this gemstone no-longer exist. The optical effect is a result of reflections from The cats-eye effect is a result of reflections from the numerous small crystals (typically copper octahedra) core of optical fibres, which are fused together to form which grow within the glass as it cools. a parallel, honeycomb-like structure within the gem.

Opalite Uranium Glass (adularescence) (fluorescence) Also called ‘Opalescent Glass’, Also called ‘Vaseline Glass’, this gemstone was favoured this gemstone glows green by Lalique for the golden glow under UV light – a result of the produced by light as it shone uranium it contains. From the through the material, and the cool 1940’s uranium became a strategic blue glow from light shining on and across it. material, leading to a pause in the gem’s production. The optical effect results from the scattering of light as The optical effect results from the chemistry of it hits areas of partial crystallisation within the glass – uranium, not it’s radioactivity: UV light is absorbed by with blue light scattered much more than red. the electrons before being re-emitted as green light.

Imitation Zultanite Carnival Glass (colour-change) (iridescence) This remarkable gemstone This vibrant gemstone first displays a vivid green in appeared in the early 20th fluorescent light, yellow in LED Century as an affordable light, an amber-orange in alternative to the ‘Favrile Glass’ incandescent light, and a purplish- jewellery of Louis Comfort Tiffany pink in sunlight. This partially resembles ‘Zultanite’; (son of the famous New York jeweller). a trademarked name for colour-change diaspore. The optical effect results from a thin layer of metal on The optical effect is a result of rare earth elements (Nd the gem’s surface which causes thin-film interference. and Pr) within the glass. A more subtle (lilac to blue) Metal films are also responsible for the play-of-colour effect is produced in ‘Neodymium Glass’. in ‘Slocum Stone’ and pleochroism in ‘Dichroic Glass’.