Our leathered granite is Via Lactea, very different from yours (black with white veining) but it did the same thing when it was first installed. I actually think for ours that it was because the granite is so dense (ie. non-porous), so the "sealer" just sat on top and the streaks were actually in the sealer. After a few weeks of cleaning with soap and hot water, the streaks went away and we have had no problems since then. It's been five years and it's pretty impervious to everything (olive oil, red wine, etc.). If you can, Google the name of your granite and see if you can find a spec sheet. The absorbency ("Water absorption" or maybe "Absorption by weight") on our granite is 0.14% which is really low. So if your granite has a similarly low number then perhaps it doesn't need sealer. :) Best of luck!