No idea what the makeup of the darker part of that rock is. The lighter part looks like a vein of something... possibly quartz? (I figure odds are it's either quartz or calcite, but I think quartz is more likely). Try scratching it with a nail/knife point. If steel can scratch it, it's probably calcite. If it won't scratch, it's probably quartz.
Calcite will raise pH and hardness, quartz is inert.