@ Markus Sachse

My personal view, they are fossilized cakes. Something inside has been broken down.

By the way, I think you would let us see the other side.

Best regard,

i just added the back site. From my impression the rounded one is completely formed with fingers out of some soft matrix, but the other peaces are carved in lithified sandstone.

Is there literature refering to something like fossilized cakes? I do not think there is organic material.

In my country China, the noodle which was made a few thousand years ago has been excavated.  

Sydney is also sandstone country.  We often see angular or odd shapes representing remnant fragments of duricrust / indurated sandstone, perhaps left as lag deposits following erosion or deflation.  This often forms as ironstone - distinctly reddish-brown - similar in colouring to one of your pieces.  Has someone selected four 'unnatural' looking pieces of smashed up natural rock, perhaps because they all contained depressions and brought them to their fireplace?  

Personally, I would love to see the discovery of fossilised Saharan cakes announced breathlessly on the news.

These appear to be completely natural pieces of sandstone with no evidence of human manufacture or modification. 

Interesting that both of you share this opinion. In fact I am not that familiar with desert weathering but I was impressed by their regular rims.

So you saw these kinds of pieces including that rounded one? About this one I still have some doubt.

Another question would be, whether there is something known about how such pieces were used (e.g. for some kind of colouring paste). They were obviously collected and brought to fireplaces. At least i was told so.