You know what it looks like… but what is it called?
TAKE THE QUIZ“Rupert's”
The following 3 entries include the term Rupert's.
Prince Rupert's metal
noun: prince's metal
Rupert's drop
noun: a congealed blob of glass formed by dropping melted glass into water and setting up such residual stresses that the globule explodes violently when the surface is scratched or a piece broken off
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Merriam-Webster unabridged