Why do crystals (like snowflakes) form geometric patterns?
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Because of the shapes of [water molecules](https://qph.cf2.quoracdn.net/main-qimg-3304fa98cc368625796eb95809b164c2) (H2O) themselves, and which parts are attracted to which other parts when you put them together, water molecules all [align themselves in a hexagonal geometric pattern](https://upload.wikimedia.org/wikipedia/commons/8/88/Ice_XI_View_along_c_axis.png) on the atomic level when they form a solid (ice). The shape of the whole snowflake is just a scaling-up of this same hexagonal structure as more and more H2Os join the crystal.
Think of it like LEGO bricks, they’re square and connect at 90 degree angles, so when you build with them the resulting shapes have 90 degree corners too (up close). Water molecules are like hexagonal LEGO bricks, they click together to form big crystals with the same 6-sided symmetry their tiny building blocks have.
While water forms a hexagonal scaffold when it solidifies, different substances (think different minerals for example) click together to form different shapes. Some make [cubic structures](https://upload.wikimedia.org/wikipedia/commons/f/fd/Pyrite_crystal_structure.png) (like [pyrite](https://hips.hearstapps.com/hmg-prod.s3.amazonaws.com/images/pyrite-crystals-in-matrix-mineral-royalty-free-image-1596210717.jpg?crop=0.898xw:0.908xh;0.0697xw,0.0612xh&resize=320:*)), some make [octahedrons](https://upload.wikimedia.org/wikipedia/commons/thumb/b/b8/CaF2_extenda.jpg/250px-CaF2_extenda.jpg) (like [fluorite](https://crystal-information.com/wp-content/uploads/2015/08/fluorite-octahedron-example-3.jpg)), etc. The shape of the bulk crystal is a scaled-up version of the atomic arrangement of the tiny building blocks, which is determined by their own size, shape, and attractions/bonding.
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