Modifiers matching glass:

• fixed,
• persian,
• transparent,
• hard,
• flat,
• opaque,
• colored,
• safe,
• decorative,
• inorganic,
• textured,
• natural,
• beautiful,
• architectural,
• cold,
• tall,
• amorphous,
• clear,
• dimensional,
• expensive,
• working,
• polished,
• typical,
• artificial,
• antique,
• inhomogeneous,
• pure,
• soft,
• small,
• optical,
• blown,
• prepared,
• plane,
• interior,
• special,
• large,
• curved,
• fine,
• black,
• hollow,
• durable,
• melting,
• warm,
• handmade,
• stained,
• broken,
• fragile,
• coloured,
• nearby,
• functional,
• under,
• resistant,
• thin,
• ornamental,
• inexpensive,
• cheap,
• industrial,
• harder,
• narrow,
• grown,

Simple categories matching harder:

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