Foam of sea water.
Meerschaum; -- called also sea froth.
Source: Webster's dictionaryAccording to one of the versions, her jealous father punished his daughter by destroying her amber palace and changing her into sea foam. Source: Internet
A variation of this story holds that they were formed from the mingling of Medusa's blood, pain and sea foam, implying that Poseidon had involvement in their making. Source: Internet