SteveD:
They are a form of structurally unstable glass similar in shape to a tadpole with an very elongated tail, formed when drops of molten glass are allowed to drop into a container of water or other liquid.
The molten glass is usually formed by heating the end of a glass rod in a bunsen burner flame.
Although I have seen the procedure performed under laboratory conditions behind a toughened glass screen, it remains a potentially dangerous procedure and serious accidents have occurred - people have been blinded and suffered severe facial burns. It is also unpredictable and you could spend all day trying to manufacture the drops without a single success. As a result, attempting to manufacture them is a procedure not to be recommended.
Why anyone would wish to sell or buy these is beyond me. They are of no useful purpose, and due to their instability and fragility they can explode into glass fragments almost spontaneously. As a result, they should not even be used as an ornament.
If one of them falls on to a hard surface the tail section invariably fractures resulting in the release of the internal pressure. This causes the "drop" to explode. Because of this, if one of them falls from your hand, you need to literally run for your life.