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Why are very old glassy rocks rare?

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Very old glassy rocks are rare because they are geologically unstable and tend to undergo a process called devitrification.

While amorphous solids like glass are formed when molten material cools too rapidly for a regular crystalline structure to develop, this state is not stable at low temperatures. Over millions of years, the atoms within the glass gradually rearrange themselves into a more stable, crystalline state. This process, which converts the glassy material into a mass of fine crystals, typically begins along cracks or around existing crystals and eventually spreads throughout the entire rock.12

Because of this spontaneous transition, most glassy rocks found in nature are relatively young, typically dating to the Paleogene period or later (less than 65.5 million years old). While it is likely that glassy rocks were abundant in the distant past, nearly all have since devitrified into crystalline forms.1