It looks like a melted marble or a piece of blown glass. I found this picture by googling melted marble. Enjoy it for its beauty. Blown glass can be quite beautiful.
Looks to me like glass to. Most probably lead glass or type of a industrial glass / product. In best case its natural glass like a tectite or bit alluvial tumbled piece of a fulgurite.
Or you stumbled over a diamond deposit
Its not that much about what youve found, its that exact moment when youve found it