Izzy,what makes you think it is a newts egg? They are usually wrapped as a single egg,around a leaf or a bit of weed.They are very small and you would normally be hard pressed to spot one.I have lots of baby newts each year but i have never spotted an egg yet.However if it really is one,then it is probably unfertilised and that is why the inside is opaque.But exactly the same can happen even in the same bunch of frogspawn,some will hatch and the rest will not.