Gem (or gemstone) is (according to dictionary (and translation to other languages like my native or japaneese)) a precious or semi-precious stone, especially when cut and polished or engraved.

So, gem is a precious stone. But not all stones are precious, ergo not all stones are gems. For example, ruby and saphire are gems, but they are also corundum, stone. Actually, ruby is red saphire, while just saphire is blue, green of violet. And of course I will not forget about star ruby (or red star sapphire) and leucosapphire (or eastern diamond). And all that gems are one type of stone, corundum.

Learn geology and play Dwarf Fortress.

Also, stoned is a status of heavy drinker, so stone-dead is more like dead because of alcohol poisoning.

Oh, and Chryssie's throne was made from magical stone, not magical gem.