Hi Kim
Putting a drop of vinegar on them and seeing if it will fizz will tell you whether or not it contains the mineral calcite, which will dissolve in water (although very slowly) and cause your
pH to rise. People who want a high pH (say for some cichlids) sometimes use crushed coral as a substrate for that reason - coral is made of calcite. The mineral dolomite is similar to calcite, but doesn't fizz when vinegar is put on it; it DOES fizz if some of the mineral is scraped off into a powder, and then some vinegar is put on that.
From what you said I think it's unlikely you've got rocks with calcite or dolomite in them (limestone). Try taking a dull steel knife and seeing if you can make a mark on the rock. If you make a mark, run your finger over it gently and see if it comes off (the mark). If you can't make a mark, try using an edge on the rock to scratch the knife - use an old knife you don't care about. If the knife scratches the rock, it's harder than the rock, and the vinegar test may work - or you may have feldspar (likely, the pink ones). If the rock scratches the knife, it's pretty likely you have all-quartz, feldspar or granite pebbles, which from your description I think seems likely.
Quartz, feldspar and granite are all inert and perfectly safe for your aquarium. Quartz is what we turn into glass, and what your substrate is likely made of (if you're using sand or gravel). Feldspar is made into porcelain. Granite is used as you know on buildings and countertops, and it's mostly quartz and feldspar.
One more thought: the 'little white lines' MAY be calcite, but if so I still think its' safe. The amount of calcite is extremely tiny, so it's effect on your pH will also be extremely tiny - you'll completely counteract that with water changes. The white lines could also be quartz.
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