The quality or characteristic of being gleamy; the state of shining brightly or having a lustrous appearance.
From 'gleamy' plus the noun-forming suffix -ness, creating an abstract noun describing the quality of the adjective. This is one of the most productive word-formation patterns in English.
The 'gleaminess' of materials is actually measured scientifically by light reflectance—when designers talk about making something look 'more gleamy,' they're using everyday language to describe precise optical properties.
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