Sea glass is human trash turned into a collectible by the ocean, and the change is as much chemistry as friction. Seawater is an alkaline solution that slowly leaches sodium and calcium from glass through hydration, leaving a microscopic crust of silica that scatters light and gives true sea glass its frosted look. Glass tumbled in fresh water, such as along the Great Lakes, is called beach glass and stays clearer, while drift glass refers to shards carried along the coast by longshore drift. A shard needs roughly 20 to 40 years to gain a basic frosting and rounded shape, and a fully mature piece can take 100 to 200 years. Color reveals the source: kelly green, brown, clear, and white are most common simply because billions of beer, soda, and juice bottles, windows, and jars were made, not because those colors last longer.

Rarer colors map to specific eras. Soft blue comes from late 19th and early 20th century ink bottles, medicines, and fruit jars, lime green from the 1960s soda boom, aqua from Ball Mason jars, and pink often from Great Depression era plates dumped by coastal towns. Red, found about once in 5,000 pieces, traces to Schlitz beer bottles, nautical lights, and early car taillights, while orange, about once in 10,000, is the rarest. Demand has fueled artificial sea glass made in commercial rock tumblers in about a month, and as packaging shifts to plastic and aluminum, the supply of new sea glass is drying up.

• Cobalt blue often comes from 19th century poison bottles whose raised warning ridges the ocean eventually smooths away.
• Opaque white usually comes from early 20th century milk bottles, and yellow from 1930s Vaseline containers.
• Antique black sea glass comes from thick 18th century gin, beer, and wine bottles strengthened with iron slag, and glows deep olive green against strong light.
• Genuine pieces keep an irregular shape, while fakes are often perfect ovals or teardrops cut before tumbling.
• The surest sign of real sea glass is a surface covered in tiny C shaped conchoidal fracture marks from countless sand impacts.