If you look at the definition of a mineral, you will find that natural ice (snow, lake ice, glaciers) is a mineral. Snow, lake ice, and glaciers also fit the definition of a rock. They are naturally occurring (not man-made), solid (not liquid, gas, plasma, etc.), and they can form large deposits. Snow, lake ice, and glaciers fit the definition, so they are rocks.
Snow is made up of many tiny pieces of ice, deposited by wind and gravity. That makes it a sedimentary rock. Keep that in mind the next time it snows. When you catch snowflakes our your tongue, you are eating a rock. When you hit someone with a snowball, you are hitting them with a rock.
As snow piles up on a glacier, it changes. The pressure of layer after layer of snow recrystallizes it into a very granular type of ice called firn. At that point, it becomes a metamorphic rock, changed by heat and/or pressure.
OK, then what about lake ice? When the surface of a lake freezes, the water changes from a liquid to a solid. Rocks that solidify from melted material are igneous rocks, so lake ice can be classified as igneous. If you get technical, it also means that water could be classified as lava. What?!?! Think about it. Lava is melted rock, right? Since snow, glaciers, and lake ice are rocks, then when the melt they form molten rock. Since it is on the surface, it is technically lava. How about a nice, refreshing glass of lava?