A thin piece of ordinary glass appears colourless to the eye, but the same glass viewed edge on will display a colour usually green, which is an indication of the presence of an iron based impurity. Impurities of other minerals can give different hues to glass, such as pink, blue, amber or grey. These properties can be used to decorative effect to produce glass of many colours and shades.
For instance, small amounts of the salt manganese dioxide will remove the green tint caused by iron, but in higher quantities the same salt will impart an amethyst hue to the glass.
Glass which has had it’s colour removed by the addition of manganese dioxide will change over time when subjected to ultraviolet light, as is found in sunlight. Over the years the colour will change through light pink or lavender to amethyst or purple subject to the quantity of the salt used in manufacture and the strength and duration of the exposure to light.
Many clear glass containers were made in the early 1900s, but I don’t find many pieces of amethyst or purple in my searches. Why? I don’t know. Maybe the glass I’m finding has been buried in sand and so not exposed to light, maybe it’s too young to have developed a hue or maybe manganese wasn’t used in it’s manufacture.
Sea glass colour rarity - RARE.