gläsern
made of glass
Consisting of the material glass.
Der Konferenztisch war modern und komplett gläsern.
The conference table was modern and completely made of glass.

glass / transparent
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made of glass
Consisting of the material glass.
Der Konferenztisch war modern und komplett gläsern.
The conference table was modern and completely made of glass.

The adjective 'gläsern' (made of glass/glass) is derived from the noun 'das Glas' (glass). The ending '-ern' is often used to describe a material something is made of.
Die gläserne Vase zerbrach in tausend Stücke.
The glass vase shattered into a thousand pieces.
In the figurative meaning 'durchschaubar' (transparent/see-through), 'gläsern' (transparent) is often used in socially critical contexts. A very common phrase is 'der gläserne Bürger' (the transparent citizen) or 'der gläserne Mensch' (the transparent person), used to describe the loss of privacy through data collection.
Viele Menschen fürchten sich davor, zum gläsernen Bürger zu werden.
Many people are afraid of becoming transparent citizens.
The meaning 'zerbrechlich' (fragile) or 'kühl' (cold) is usually recognized in connection with emotions or moods. Typical examples are 'ein gläsernes Lächeln' (a fake, fragile smile) or 'gläsernes Schweigen' (a cold, tense silence).
Sie gab ihm nur ein kurzes, gläsernes Lächeln.
She gave him only a brief, fragile smile.
cold
'kalt' and 'gläsern' can both describe, in a figurative sense, an effect without warmth or closeness. 'kalt' is often used for people, voices, or behavior when someone seems distant, unfriendly, or emotionless. 'gläsern' is more specific and more figurative. It can describe something fragile or a cool, almost transparent effect, for example in a look or a voice. For ordinary unfriendly behavior, 'kalt' is more natural. 'gläsern' fits better when the impression is meant to be described as especially delicate, fragile, or unusual.

Nach der schlechten Nachricht klang ihre Stimme gläsern und fremd.
After the bad news, her voice sounded glassy and strange.
Bei dem Gespräch blieb ihr Ton kalt und höflich.
During the conversation, her tone remained cold and polite.