I am pretty sure this is supposed to be magma blocks.
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Thank you for your report!
However, this issue has been closed as Working as Intended.
These newer blocks will be fully available (with their correct textures) in an upcoming update (they were added in 1.1.3.1 beta. Until then, they will show as "Update!" blocks.
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Can you please verify and add the correct game version that you are using?