Simple categories matching harder:

• medium,
• stone,
• material,
• genre,
• food,
• surface,
• thing,
• part,
• polymer,
• metal,
• aggregate,
• drug,
• crystal,
• currency,
• agent,
• cheese,
• narcotic,
• deposit,
• layer,
• stuff,
• stimulant,
• wood,
• ingredient,
• rock,
• substance,
• feldspar,
• boot,
• fabric,
• structure,
• sediment,
• ground,
• mineral,
• pin,
• tissue,
• object,
• sport,
• resin,
• particle,
• powder,
• abrasive,
• filler,
• glass,
• music,
• alloy,
• formation,
• work,
• assets,
• substrate,
• gemstone,

Members with property harder:

• metal,
• granite,
• black_hole,
• ecstasy,
• grout,
• sink,
• sapphire,
• lower_part,
• pen,
• chalk,
• soccer_field,
• butter,
• steel,
• stone,
• emerald,
• bones,
• quartz,
• diamond,
• table,
• polystyrene,
• gravel_path,
• hardwood_floor,
• beryl,
• marble,
• stainless,
• carbide,
• sintered_diamond,
• dental_stone,
• tin,
• hardest_sandstone,
• paved_street,
• dike,
• carving_wood,
• zinc,
• peridium,
• machinery_mount,
• gravel,
• cornelian,
• hard_rock,
• metamorphic_rock,
• academic_standard,
• shoes,
• cocaine,
• consolidated_earth,
• floating_timber,
• chitinous_exoskeleton,
• emmental,
• soft_steel,
• vinyl_tile,
• cocaine_coke,
• methadone,
• epoxy,
• jack,
• dime,
• frozen_ground,
• metal_carbide,
• testing_accountability,
• book,
• turtle_shell,
• hematite,
• topaz,
• sandstone_aggregate,
• clutch,
• borosilicate_glass,
• cement,
• joint_cartilage,
• siding,
• sharpening_stone,
• edge,
• gun,
• apple,
• piling,
• track,
• insect_part,
• platinum,
• bone,
• chrysoberyl,
• ice,
• amalgam,
• brass,
• quality_cheddar,
• flint_powder,
• caffeine,
• fire_trail,
• lava,
• cobalt_chrome,
• cobble,
• plastic_brace,
• plastic,
• carnelian,
• stove,
• pottery,
• calcium,
• arsenopyrite,
• diamond_powder,
• techno,
• silica,
• hope,
• york_stone,
• semiconductor,
• palladium,
• water,
• zirconia,
• exposure,
• titanium_carbide,
• dollar,
• chlorite,
• tire_track,
• whinstone,
• tarmac,
• coke,
• barrack,
• pure_polyurethane,
• hair,
• joint,
• tungsten,
• crust,
• extractor,
• organs,
• haematite,
• super_glue,
• pcp,
• bauxite,
• polystyrene_resin,
• twinkie,
• silicon,
• chrome_plating,
• speed,
• denim,
• cotton,
• sapsago,
• nail,
• silicon_nitride,
• tungsten_carbide,
• pure_titanium,
• teeth,
• acrylic_cement,
• torso,
• corneal_stroma,
• parmesan_cheese,
• mozzarella,
• gold,
• aluminum_nitride,
• barbiturate,
• watercolor_papers,
• nylon,
• flint,
• zircon,
• wall,
• alloyed_aluminium,
• aerospace_alloy,
• liver_pate,
• crushed_garnet,
• celery,
• clay,
• aquamarine,
• paint,
• yoghurt,
• rose_gold,
• armoured_steel,
• computer_case,
• sidewalk,
• silver,
• articular_cartilage,
• slate,
• pyrite,
• small_tree,
• pu,
• plaster,
• sapphire_ball,
• chalcedony,
• piece,
• stucco,
• garnet,
• copper_alloy,
• goal_post,
• pulverised_coke,
• gel,
• walnut,
• lime,
• magnesium_sand,
• cheddar,
• fabulous_aluminium,
• car_usage,
• macadam,
• wood,
• feldspar,
• amethyst,
• jelly,
• copper,
• basketball,
• geography,
• knot,
• palladium_alloy,
• fiberglass,
• corundum,
• hornblende,
• tee_box,
• vein,
• shoe,
• italian_cheese,
• tranquilizer,
• kidney,
• polyurethane,
• bronze,
• tube,
• siliceous_gravel,
• treadmill,
• bone_fragment,
• pavement,
• metal_plate,
• volcanic_rock,
• plywood,
• hard_acrylic,
• cold_butter,
• titanium,
• punk,
• acorn,
• magnetic_rubber,
• grinding_stone,
• brick,
• gruyere,
• low_carbon,
• harder_marbles,
• output_shaft,
• carrot,
• american_dollar,
• fresh_basalt,
• alloy,
• bamboo,
• ash,
• boat,
• almandite_garnet,
• polyamide,
• ground,
• bone_cement,
• asphalt,
• molybdenum,
• tile,
• nickel,
• obsidian,
• amphetamine,
• trap_rock,
• mica_slate,
• face,
• ruby,
• brazilian_agate,
• diorite,
• alabaster,
• quartzite,
• engine,
• quartz_powder,
• illustration_board,
• hard_ground,
• horn,
• cartilage,
• skin,
• aggregated_diamond,
• cleaning_toilet,
• white_gold,
• gunmetal,
• opiate,
• nitride_barrier,
• portfolio_cover,
• shell,
• stimulant,
• then_asphalt,
• chert,
• architecture,
• mild_steel,
• aluminum,
• parmesan,
• pencil,
• basement_gneiss,
• skeleton,
• metallic_paint,
• agate,
• dock,
• aluminium,
• pvc,
• turquoise,
• basalt,
• newspaper,
• jade,
• heroin,
• inconel,
• od_silver,
• sand,
• septal_cartilage,
• concrete_roads,
• carborundum,
• boot,
• fact,
• compound_semiconductor,
• dense_muscle,
• roads,
• coconut_shell,
• disappointment,
• input,
• acrylic_acetate,
• schist,
• concrete,
• pyrites,
• sunset,
• tv,
• pill,
• floor,
• metal_mesh,
• hard_wood,
• titanium_alloy,
• quartz_sand,
• linoleum,
• alkyd,
• iron,
• quarters,
• hq_till,
• dolomite,
• maple,
• masonry,
• lsd,
• point,
• carboniferous_limestone,
• rock,
• chrome,
• decoration,
• glass,
• container_housing,
• pebble,
• head,
• limestone,
• gneiss,
• crack,
• shale,
• leicestershire_granite,
• acrylic,
• meth,
• seed_shell,
• claw,
• cobalt,
• calcium_deposit,
• rutile,
• quartz_grain,
• glacial_till,
• rubber_pole,
• snowboard_boot,
• andesite,
• artery,
• hardwood,
• ceramic_tile,
• heart,
• crystal_meth,
• onyx,
• sycamore,
• mojo_helmet,
• ionic_material,
• stem,
• marijuana,
• concrete_pavement,
• road,
• phenolic,
• soft_wood,
• methamphetamine_speed,
• gun_metal,
• alumina,
• sandstone,
• donut,
• webbing,
• steel_alloy,
• mushroom,
• opal_stone,
• radio,
• chromium,
• fibrous_tissue,
• cable,
• print_magazine,
• sawing_wood,
• gouda,
• tourmaline,
• muenster,
• jasper,
• animal_bone,
• wyoming_feldspar,
• cricket_table,
• wood_floor,
• taconite,
• alcohol,
• potato,
• crystal_methamphetamine,
• spicule,
• flint_stone,
• gallium_arsenide,
• yellow_gold,
• paved_roads,
• solid_rock,
• ligament,
• citrine,
• rap,
• particulate_char,
• crystal,
• nutmeg,
• methamphetamine,
• tendon,
• shell_hash,
• corundum_ruby,
• emmenthal,