Ice as a stone
Ice appears as a light blue stone which can be found by mining through an ice wall (such as that created when a river freezes). Ice boulders and objects made of ice will melt after some time when exposed to warmer temperatures (such as inside a fortress), giving it rather limited use; ice boulders can even melt while being carried by your dwarves, causing them to drop the item and resulting in a smear of water on the floor beneath them.
It is possible to build constructions out of ice, even inside a fortress (provided your dwarves move quickly enough to finish construction before the ice melts). These constructions are then stable even as temperature rises. Pillars of ice walls can even be built within rooms that are later flooded with magma (though the walls become "warm", they will not melt).
Attempts to build workshops, paved roads, or other non-construction buildings out of ice will usually result in the ice melting immediately, a message "The dwarves were unable to complete the [workshop_name]", and the workshop construction being canceled completely. The exception is when building in a glacier environment the construction can be completed. In winter in temperate to cold environments, it is also possible to build these; however, they will melt when the usual ice also melts.
Ice cannot be used to craft. It's not possible to create doors, hatches, and other crafts from it.
The game refers to ice boulders as "water". It does not appear in any stockpile options or the manager, so it cannot be moved by designating a stockpile. Ice can be moved by dumping it. When an ice boulder melts inside a fortress, it becomes an item named "water", but it differs from regular water: it has no liquid level, does not leave mud when it evaporates, and dwarves eventually remove it with "clean" job. Melted ice is of no value as a water source.
A block of ice is worth 3☼ if traded, and weighs 92Γ.
 Observations on constructing with ice
You only get 1 fill pond job per pond zone, so it works best to make many of them, such as a series of 10x1 zones. Once the pond zone is finished, you will start getting notifications that they cannot fill the pond. At this point, you can just delete the zone and the fill pond job goes away on its own.
It takes 2 buckets to make a tile of solid ice. The first bucket makes an ice floor, while the second bucket makes the ice wall on the level below.
Make your water source as close as possible to the construction site. It needs to be underground so it doesn't freeze, but closer means things get cast from ice more quickly.
Water in buckets will last for a surprisingly long time before freezing in the bucket as a chunk of ice. It's really not an issue unless the bucket is dropped and left outside. If the water in the bucket does freeze, just order the ice to be dumped, and the chunks of ice will be taken to your garbage zone.
Put your bucket stockpile right next to water source and produce a large number of buckets to ensure there is always another bucket to be filled.
You can cast ice over a ledge. First the ice floor and then the ice wall is cast at the Z-level right below the pond zone. This means if you're dumping buckets of water off of a 2 Z-level high wall, you will be forming a solid ice roof and the area underneath will be completely ice free, and even safe to walk over as the ice is being cast over their heads.
You can cast ice walls right to the edge of the map. Effects on creatures spawning there is unknown, currently undergoing testing.
 Thawing and drowning
In fortress mode, if a frozen murky pool thaws, it will do so instantly. Any dwarf standing on the ice will plunge into the water and drown (unless they have Adequate or better swimming skill, the fall will stun them). To reduce the chance of this happening, you can use traffic designations: mark the ice as restricted, and put a high traffic ring around the perimeter of the pool. Or, if you don't need to fish from it, you can construct a wall around it or a floor over it.
Running magma under unmined ice will melt the ice. This appears to be the best way to turn large amounts of ice into water to use as a water source when all you have is ice and magma.
 Constructing around ice
Using the smooth stone and carve fortification designations work as normal with ice. However, if an aquifer or other source of water is present next to your new fortification, it will immediately freeze into an ice wall when the water flows into it.
The freezing/thawing of ice deconstructs anything occupying the same space as the block of ice, including floor hatches, grates and wells built over over it.
If a structure is supported by ice which then melts seasonally, the game does not check to see if the structure should collapse, resulting in a building hovering over the now-melted ice.