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Item: Single-stone pendant with fixed bail.

Metal: Sterling Silver 925 (presumed based on visual)

Gemstone: Natural Lemon Quartz

Cut: Octagon (likely an emerald or step-cut variant)

15mm 
Total Weight: 42 carats (or grams, unit clarification required)

Just to confirm: Is the weight you provided (42) in carats (which is common for gemstones) or grams (which is common for jewelry)? I used carats in the descriptions above as 42 grams is quite heavy for a small pendant.