Trash thrown into the sea is a global and unfortunately ever-increasing problem. Most of the garbage is plastic because the decay of this material takes enormous time. Another type of resistant trash dumped into the sea is glass.
A very small part of it may indeed come from castaway sailors sending bottle messages but mostly it is the result of people using sea as a huge garbage dump. Glass is not as visible and annoying in the sea as plastic because it is heavier than water and sinks to the bottom. And rounded glass pebbles can easily be mistaken for some minerals. It may look very similar to quartz, especially if the glass is transparent.
In our particular case the transparent grains are indeed glass pebbles, not quartz. It is easy to distinguish between the two with the help of a polarizing microscope. Unlike quartz, glass has no crystal structure and is therefore dark (showing no interference colors) when placed between the two crossed polarizers of the microscope.
Sea glass has lost its sharp edges because of the tumbling action of waves. It is frequently washed ashore but the concentration of seaglass in the beach sand is usually very low. There are some notable exceptions such as the Glass Beach in Kauai Island or the Glass Beach of Fort Bragg in California. Glass has concentrated there because these places were used as garbage dumps in the past.
I also think that ocean-weathered glass is beautiful, including sometimes bottles on the beach. Might the ocean serve as a way to recycle glass, to turn it back into sand?!! Plastic trash, on the other hand … yuk!!