We have hard water from our well. It is filtered twice before boiling it: by the water filter and with a manual on-the-counter filter. When boiled in a clean kettle a white substance floats in the water, and it isn't calcium which is hard and sinks at the bottom. Then we filter that hot water through a coffee paper filter and a white creamy substance remains. I called water related companies but no one knows. Anyone have a answer to this one?
I assure you it is calcium carbonate.
The particles are so small they do not just sink to the bottom like a stone. Instead, they aggregate at the nearest phase boundary. All small impurities do that, unless they have a very low interfacial tension with water. It's energetically much more favourable than sinking.
Over time, this creamy substance aggregates further, up to a point where the aggregates are large and dense enough to dissociate from the surface, and sink to the bottom.
Hard water often contains magnesium as well as calcium. A ppt of Mg(OH)2 or Mg2CO3(OH)2 would give a pH ~10 (Tee's result).
The ppt could be a function of the pH of the water; temporary hard water contains bicarbonates which are more soluble than the carbonates. When the water is heated, it drives off CO2, which could drive nano particles to the surface until they grow and fall.
Try dissolving the creamy ppt with acid (vinegar or HCl). That would give some indication of metallic nature. To be complete, you could try adding household ammonia (no soap in it) to see if you get dissolution (clarification of the liquid).
You could also take the twice-filtered, but cool, water and add sodium bicarbonate to see if you get a ppt. Then take the supernatant and boil it - there should be no more ppting. And likewise, to be complete, try adding a little vinegar to get a pH ~ 4-5; this would likely keep Ca and Mg in solution.
Magnesium gives no color in a flame test. https://en.wikipedia.org/wiki/Flame_test