Simple categories matching harder:

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

Members with property harder:

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