Hello

The other day a blue stone was handed to me for identification. It 
 was found in the intertidal zone in Mort Bay, Sydney Harbour,
Australia..

 I have never seen anything like it - either from Mort Bay or from
anywhere.

 https://app.box.com/s/6da0yfp1w9bzaitei347ltogwd1s9rux

 The intertidal zone has gravels that include exotic rocks, such as 
 flint. These are interpreted as discarded ballast from ships. Indeed 
 the point on the other side of the bay is called Ballast Point,

 The blue stone is much abraded, presumably by sand in the intertidal 
 zone. There are what appear to be inclusions of ironstone embedded in
the blue matrix.

 My provisional conclusion is that it is slag from smelting iron, akin

 to the process that produced the blue slag from Leland in Michigan,
USA.

 http://mynorth.com/2014/08/leland-blue-a-way-of-life-in-northern-michigan/

It has been suggested to me on another forum that there are two other
possible interpretations of this piece of blue stone:

1. It is silica that accumulates in the ashes during the refining of
sugar.

2. That some black bottle glass turns blue in patches when subjected
to fire.

I am inclined to stick with my provisional conclusion that it is slag.
This is because of the inclusions of seemingly ironstone pieces
embedded in the blue matrix.

Any suggestions will be gratefully received?

 Richard