what is this ?

[goofball]

saw this while wandering around bowl of fire in lake mead. thought at first glance a lithic, but a closer look and it seemed to "flaky" for a rock, even though it is "rock hard". then thought maybe it was a tooth ? the edges are awfully serrated, to serrated to be any natural weathering. any ideas ?

[basilone0331]

Hard to tell from the picture, but it looks similar to white opal I've picked up outside of Milford.

I googled white opal and this was the best looking image of the type of rocks we were finding.