Sea glass aka Beach glass
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Sea glass in several colors and shapes.
Sea glass (also known as beach glass, mermaid's tears, lucky tears, sea gems and many other names) is glass found on beaches along oceans, bays, sounds, rivers or large lakes that has been tumbled and smoothed by the waves, water and sand, creating smooth, frosted shards of glass.  
Sea glass is one of a very few cases of a valuable item being created from the actions of the environment on man-made litter.
a piece of red sea glass, one of the rarest colors
An old bottle top made of cobalt blue glass, one of the rarer colors
The color of sea glass is determined by its original source. Most sea glass comes from bottles, but it can also come from jars, plates, windows, windshields, glasses, art, flasks, containers, and any other glass source that has found its way into the ocean. Some collectors also collect beach ceramics or sea pottery.
The most common colors of sea glass are kelly green, brown, and clear. These colors come from bottles used by companies that sell beer, juices, and soft drinks. The clear or white glass comes from clear plates and glasses, windshields, windows, and assorted other sources.
Less common colors include jade, amber (from bottles for whiskey, medicine, spirits, and early bleach bottles), golden amber or "amberina" (mostly used for spirit bottles), lime green (from soda bottles during the 1960s), forest green, and ice- or soft blue (from soda bottles, medicine bottles, ink bottles, and fruit jars from the late 1800s and early 1900s, windows, and windshields). These colors are found about once for every 25 to 100 pieces of sea glass found.
Uncommon colors of sea glass include green, which comes primarily from early to mid-1900s Coca-Cola, Dr Pepper, and RC Cola bottles, as well as beer bottles. Soft green colors could come from bottles that were used for ink, fruit, and baking soda. These colors are found once in every 50 to 100 pieces.
Purple sea glass is very uncommon, as is citron, opaque white (from milk glass), cobalt and cornflower blue (from early Milk of Magnesia bottles, poison bottles, artwork, and Bromo-Seltzer and Vicks VapoRub containers), and aqua (from Ball Mason jars and 19th century glass bottles). These colors are found once for every 200 to 1,000 pieces found.
Rare and extremely rare colors include gray, pink (often from Great Depression era plates), teal (often from Mateus wine bottles), black (older, very dark olive green glass), yellow (often from 1930s Vaseline containers), turquoise (from tableware and art glass), red (often from car tail lights, dinnerware or from nautical lights, it is found once in about every 5,000 pieces), and orange (the least common type of sea glass, found once in about 10,000 pieces). These colors are found once for every 1,000 to 10,000 pieces collected. Some shards of black glass are quite old, originating from thick eighteenth-century gin, beer and wine bottles.
Like collecting shells, fossils, or stones, 'combing shorelines for sea glass is a hobby many beach-goers and beachcombers enjoy. Hobbyists often fill decorative jars with their collections and take great pleasure in sourcing out a shard's original origin. Artisans craft beautiful, much sought after pieces of jewelry, stained glass and other decorative items from sea glass.
Sea glass can be found all over the world, but the beaches of the northeast United States, California, northwest England, Mexico, Hawaii, Puerto Rico, Nova Scotia, Italy and southern Spain are famous for their bounty of sea glass, bottles, bottle lips and stoppers, art glass, marbles, and pottery shards. The best times to look are during spring tides and perigean and proxigean tides, and during the first low tide after a storm.
Beach glass, as it is called on inland waterways such as Chesapeake Bay and the Great Lakes, though similar to sea glass, may be less weathered because such waterways lack wave rigor and the saline content of oceans. Beach glass from these regions often have very clear embossed designs or letters on them, which makes tracing their origin easier. Shards may also evidence a frosted side and a shiny side, most likely because they are pieces broken off from larger glass objects still embedded in mud, silt or clay, which are only slowly being exposed by wave action and erosion.
Authentic sea and beach glass is becoming harder and harder to find for a variety of reasons: there are more people searching for it; glass items have been replaced by plastics; and littering is discouraged.
Some artisans and crafters have taken to tumbling less-than-stellar pieces of sea glass shards to create what is called "twice-tossed" glass. Others rely on artificial sea glass, produced to meet craft demand at a cheaper price and in a wider range of colors, which is also often produced using a rock tumbler. Such glass is chunkier than most true sea glass, lacks its' romantic provenance, and differs in many technical ways, (e.g. long-term exposure to water conditions creates an etched surface on the glass that cannot be duplicated artificially).
For many professional sea glass collectors, authors, artisans, and retailers, the main issue is honesty regarding the source of glass. While some prefer the term "twice tossed" or "craft glass" for inauthentic or artificial products, this usage is by no means universal.