Some black sea glass can be mistaken for common stones to the untrained eye of a novice beachcomber.
Pick up the object in question and hold it in front of the sun to determine if it is a sea glass gem.
Finding Black Sea Glass
Black sea glass can easily be mistaken for an oridinary stone
Because black sea glass is generally very dark and chunky it can look like an ordinary stone. The untrained eye of a novice sea glass collector may overlook such rare gems. It is hard enough finding an area that is old enough to yield such precious sea glass; don't let such a special find slip by you!
If you come across a dark and chunky, stone-shaped object, take the time to investigate. Pick it up, wet it and hold it in front of the sun. The strong light from the sun will, in most cases, reveal any translucency. The light will also reveal the color of the shard, usually an olive-green or olive-amber. Wetting the sea glass helps reduce the dispersion of light by the frost and pitting and thus allows the light to pass more readily. If it is a cloudy day bring a small flashlight with you.
To see more on black sea glass click August Shard of the Month
In some instances the glass is so dark it may only reveal its true nature around the edges. I have picked up glass that just did not let any ambient light through. Generally, if it is glass, it will be smooth and have a frosted surface. At that point you have to just trust your instincts and keep it until you can properly confirm its rightful identity.
With these techniques you will begin to develop a keen eye for recognizing the differences between the dark stones generally found on a particular beach and what might possibly be a black sea glass shard.
There were plenty of times that I almost didn't stoop to pick up a dark colored piece but did, and was rewarded for my efforts.