Luster
How a mineral surface reflects light. Metallic lusters (galena, pyrite, hematite, chalcopyrite) belong to the sulfides and native metals — the surface looks like polished iron. Sub-metallic is duller (some hematite, some manganese oxides). Non-metallic lusters branch out: vitreous (glass — quartz, calcite, fluorite), resinous (sphalerite, sulfur), pearly (talc, gypsum, micas), silky (asbestos, gypsum satin spar), greasy (nepheline, opal), adamantine (diamond, cerussite, anglesite), and dull / earthy (kaolinite, bauxite).
Streak
Drag the mineral across an unglazed white porcelain plate. The line of powder it leaves is the streak — the mineral's color in fine-grained form. Streak ignores body color and shows the true diagnostic. Hematite is silver-black externally but always leaves a brick-red streak. Pyrite is gold but streaks greenish-black. The streak plate is the most-underrated tool in a collector's kit.
Cleavage and fracture
Cleavage is breakage along planes of atomic weakness — perfectly flat, repeatable, mirror-like. Calcite shows three cleavages at 75° / 105° — the classic rhomb. Micas have one perfect basal cleavage — they peel into sheets. Galena has three cleavages at right angles — perfect cubes. Where there's no cleavage, you get fracture: conchoidal (curved, shell-like — quartz, obsidian), splintery (asbestos), hackly (native copper), uneven (most everything else). Combined with hardness, cleavage usually nails the ID.