Heavily Fortified Agate (bold lines).
This pattern reminds me of a dolphin or whale.
This is a butt end so it has a natural back, but it is a shallow butt end as per the photos of the side. This allows you to backlight it and see the patterns through it which it backlit.
From Barossa, Mt Somers, Canterbury, NZ
Weight: 264 grams
Measurements: 145 x 79 x 16 mm deep