Of Paving & Crystals ~ Mario Freitas of Universidade Tecnológica Federal do Paraná was walking along a sidewalk in Curitiba, Brazil when he noticed regular dark and white 'rays' or lines radiating across the paved ground. They are most apparent by looking at the top of the image and blurring the eyes slightly.   ©Mariao Freitas
Three of the several possible forms of cubic lattice crystals.  Facet directions follow free energy favoured planes through the lattice.

   cube - rock salt
   octahedron - fluorite, alum
   cuboctahedron - fluorite, galena

Grow your own.
Deliberatly blurred to smooth out the brick joins. Some extra lines running from left to right now show up, probably due to the different darknesses of individual bricks.

The paviers or bricks form a periodic pattern. In this case the bond is fairly complicated - more common ones for paved areas are at right.

Look for the symmetry elements and think how you would start to lay the Brazilian one.

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The dark and light rays arise as the eye looks along directions of greater or lesser density of dark joins.

The yellow line is along a high join density - that direction will look dark. The small square bricks are shaded to emphasise the pattern. In reality the brick colours and tones are not systematic and do not generate the lines.

Look for patterns on siewalks, wall, tiles roofs.. ..everwhere.

This image is processed to leave only the joins between the paviers and to totally ignore their colours and tones. The dark lines are even more evident. Somehow the dark brick joins are responsible.

Crystals are 3D periodic lattices of atoms, ions or molecules.

The solid lines across this simple cubic crystal define lattice planes - if projected into 3D - of high atom density. Other planes exist in three dimensions.

Crystal faces follow lattice planes of high density as do mineral and gemstone cleavage planes.

Directions of lower density like the dotted one are less likely to form facets.