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Hi there, I found this rock in the surf somewhere on the east coast of Australia around 20 years ago. Not sure if this is sea glass or a rock but hoping someone can help me identify as it has bugged me for a long time.

Surface is Very smooth and it is a dark green / black colour on one side and translucent white on the edges. Some light filters through the rock which makes me think this is some type of glass object that has Been weathered by the ocean over many years although it’s quite big, solid and heavy which is unusual for glass what has me confused.

It is on the heavy side weighing 69g and is 6cm long and 2.4cm wide at it’s shorter end whilst expanding to 4.5cm wide at its widest point. It is 1cm high at the short end and 2cm at the wide end.

Please let me know if you require any more info


Tboneenter image description hereenter image description here

  • 1
    $\begingroup$ Hi Tbone, it might help the experts if you can answer some additional questions in our rock identification questions guide. Cheers :) $\endgroup$ – JeopardyTempest May 13 '18 at 1:18
  • $\begingroup$ Thanks for doing it. Far too many don't! I am hopeful you'll find a lot of good input :) $\endgroup$ – JeopardyTempest May 13 '18 at 18:14

It appears to be chert or in your case flint, as the darker type is called. There are two ways chert may form either by the growth of silicon dioxide microcrystals within softer sedament which may form limestone. The other way and the most probable for your stone is biological in origin. The glass like silicon skelotons of ancient diatoms a one cell animal which is found in shallow seas and in oceans, died and sank depositing layers which redisolved and formed chert or flint


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