Can you feel the lines? Are they the same texture? This doesn't sound normal, but natural stone can have variations like that. Is it like a cleaning streak? I would give your fabricator a call and see what they say. Having worked with the material, they will know it best. If they don't give you a satisfactory answer, then post on here again and we will go from there! Good luck.

I was looking up the geological properties of quartzite, and I saw this: "When sandstone is metamorphosized to quartzite, the individual quartz grains recrystallize along with the former cementing material to form an interlocking mosaic of quartz crystals.[2] Most or all of the original texture and sedimentary structures of the sandstone are erased by the metamorphism.[2] The grainy, sandpaper-like surface becomes glassy in appearance.[2] Minor amounts of former cementing materials, iron oxide, silica, carbonate and clay, often migrate during recrystallization and metamorphosis. This causes streaks and lenses to form within the quartzite."

Notice the last sentence. I don't know if this is relevant to your exact situation, but it might help.