The term "adamantine" generally refers to something that is hard, unyielding, or glass-like in appearance. Based on your options:
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Looks like glass: This interpretation emphasizes the glass-like quality, suggesting transparency or a smooth, reflective surface, which aligns well with the term's associations of hardness and brilliance.
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Looks like amber, not quite glassy: This option suggests a softer, warmer appearance, reminiscent of amber's semi-translucent, organic qualities. While it may not be as hard or reflective as glass, it still conveys a sense of beauty and richness.
Depending on the context, either option could work. If you want to emphasize a more crystalline, reflective quality, "looks like glass" would be the better choice. If you're aiming for a warmth and a more organic feel, "looks like amber, not quite glassy" might be more appropriate.