It depends on what recipe you use. Just tried to weigh two of mine, 5" X 5". Two different recipes and different thickness, 1/2" and 3/4". The 1/2" was made using a vermiculite recipe and weighed about 3/4 pound. The 3/4" made with peat moss weighed about 1 pound. There is also a little ice on both so that would add a little to the weigh. When it rains they'll also absorb water and weigh more.

Concrete weighs about 140lbs per cubic foot. Tufa and lightweight concretes vary quite a lot, but half the weight is a good rule of thumb. You won't want to use any of the really light mixes for a thin plaque because they'll be too weak.

So your 6"x10"x3/4" in concrete would weight about 3 1/2lbs. In hypertufa or a strong lightweight concrete, about 1 3/4 lbs. The 4"x4"x1/2" would be a little over half a pound in concrete, a quarter pound in a lightweight mix.

You can make lightweight mixes easily half that light again if you are not concerned about strength. You would need to mount them on a supporting surface and not expose them to any wear or weathering.

these will be indoor at all times, i am trying to fashion a 'stone' top that would be the lid to a wooden box. It would be supported by a light, thin wooden surface. I plan to seal them also after a bit of polishing if possible.

So, would the lightest then be the lightweight mix, but tweaked some more? I'll look in the forum for lightweight recipes.

dp