I need more information than that to even tentatively identify it. (Note the correct spelling of insoluble.)
Apparently it smells really bad when you burn it. I don't know if that helps.
Colloidal sulfur is white and flacky. It burns producing sulfur dioxide which isn't pleasant to smell but none of that proves anything.
I have to figure out whether the substance is ionic, covalent, molecular or atomic and all I know is that it is insoluable in water, that it is white and flaky, it has no electrical conductivity in water and it's brittle. I don't know how to go about figuring out what kind of substance it is.
Did you do any more tests on it. Write what you did instead of having us guess what you may have had out of thousands of compounds and with meager information. No electrical conductivity probably means it isn't ionic and that fits with poor solubility in water.
I only did 3 tests. Hardness, conductivity, melting point. Supposedly we are supposed to be able to figure out what type of substance it is from those three tests but I'm still uncertain.
Ionic compounds usually are soluble in water, they usually are brittle, and they usually conduct electricity. Molecular compounds usually don't dissolve in water, they don't conduct electricity, and usually they are soft. But if those three tests are all you have to identify the compound (perhaps you have some choices of compounds), it's easy to make an error BECAUSE some ionic compounds are not soluble and if they don't dissolve there can be no conductivity. And diamonds, for example, are extremely hard but they are the epitome of covalent (networking).
Thanks so much :)