What do I have here?

[Yiop]

 

[grubstake]

Looks like frothy, iron-rich basalt to me, but I'm not a geo. Where did you find it?

 

[Hawkear]

Basalt does occur with gas voids but usually they would be roundish, whilst these seem to be be very sharp and angular. Almost as if some crystals have been dissolved out of the specimen during weathering.
Is that quartz along the top?
Perhaps a location as Grubstake suggests would be helpful as the types of rocks that generally host quartz veining can differ a lot throughout Australia

 

[Yiop]

North East Victoria, Yes quartz along the top

 

[Hawkear]

I have seen similar colouration and texture in rocks i found in the Creswick area. They seemed to be vein wall ironstone excavated from an old nearby mine.
I found a large piece and several smaller pieces when I got a signal from them. To my surprise when I examined them more closely I saw that they were indeed specis with small amounts of quartz and some small nuggets enclosed.
I assumed the variation in colouration of the ironstone to be caused by weathering activity. I haven't retained any but from memory I think they also had voids like those shown, perhaps caused by other mineral crystals or inclusions being weathered out of the ironstone completely.
If these are of similar character maybe check the area out a bit more.
Have also seen basalt weather to the same sort of colouration but the shape of the voids would worry me there.

 

[user 4386]

Basalt does occur with gas voids but usually they would be roundish, whilst these seem to be be very sharp and angular. Almost as if some crystals have been dissolved out of the specimen during weathering.
Is that quartz along the top?
Perhaps a location as Grubstake suggests would be helpful as the types of rocks that generally host quartz veining can differ a lot throughout Australia

Looks like vuggy silica, the cavities perhaps representing something like calcite or another mineral.

 

[AKA_RedRimmed_Desert]

Find them with a gold detector?