Quoted from wikipedia:
Sculpting ice presents a number of difficulties due to the variability and volatility of the material. Ice must be carefully selected to be suitable for the sculptor’s purposes and should be free of undesired impurities. Typically, ideal carving ice is made from pure, clean water. However, clear, transparent ice is a result of the freezing process and not necessarily related to the purity of the water. Clear ice is mostly the result of slow freezing that allows impurities to escape. The water molecules are allowed to line up into the typical crystal lattice while impurities remain in the unfrozen water. Certain machines and processes allow for slow freezing and the removal of impurities and therefore are able to produce the clear blocks of ice that are favored by ice carvers. However, not all blocks that are carved are clear ice. White ice blocks look like snow and are sometimes carved. Colored ice blocks are produced by adding dyes to the ice and can be carved as well. In some instances, clear ice and colored ice are combined to create a desired effect.
Frozen in thought: Ice sculpture entry for Zehnder’s Snowfest, Frankenmuth, Michigan USA (taken in Jan. 2008 by David :0)
Detail of ice sculpture of child: Halifax Ice Sculpture Trail, Nova Scotia, Canada (taken in Dec. 2008 by john2544)
Ice Queen: from Wereldwonderen, Netherlands (taken in Dec. 2007 by Eisbeertje)
Cold Icy Stare: Exposición de Esculturas de Hielo in Madrid, Spain (taken in Dec. 2007 by tonymadrid)
Royal Icy Treatment: Ice Sculpture Festival and Trail, Norwich, Norfolk, UK (taken in Dec. 2005 by Leo Reynolds)
Ice Fantasy: International Competition of Ice and Sculpture in Khabarovsk, Russia (taken in Jan. 2008 by smakogon)