Rock Check!

River rocks should be fine to use, not sure about the yellowish one. It might just be a river rock as well.

what you do is test them. use the nitrate #2 bottle from your API test kit and place a drop on the rock, if it bubbles up then you have a rock that will raise the ph of your tank and throw it out. if no bubbles then its a "neutral" rock and safe to use. you can also use vinegar if you don't have an API test kit but vinegar doesn't show the bubbles as well.

Based on what I can see in the photos you look to have a mixture of granites possibly gneiss, and possibly some sandstone and mudstones. As stated above get some acid to betermine if they contain Ca [lime stone]. As far as the Shiney flakes they are most likely mica, since you have some igneous and metamorphic rocks. Don't worry about them as they are basic silica minerals.

One additional thing just because it's a river rock does not mean it's safe. I know places where the rivers are dead because of the rocks that are in them. Never assume just because the rock is in the river it's inert.

Have a gniess day. -Scott