A glass ionomer ( GIC ) is another type of tooth coloured material increasingly promoted as a substitute for dental amalgam in children as well as lining materials under plastic fillings i.e. composite resin materials. They are silicaceous gels made up mainly by mixing an alumino-silicate glass powder with a polyacrylic acid. 

In general their physico-mechanical properties are inferior to either amalgam or composite resins so they are usually used in a sandwich technique with composite resins so that the weaker material  ( GIC ) is covered with a layer of more resistant composite resin. Many manufacturers have mixed various amounts of resins with glass ionomers in an attempt to improve their physico-mechanical properties and these " hybrids " are marketed as resin-modified glass ionomers or compomers.

Notwithstanding the rational for their use, and, like amalgam, their proponents have many  " good " reasons to use them. Glass ionomers and resin modified glass ionomers have highly questionable biocompatibility and contain various species of fluoride, aluminium and, frequently, contaminants such as lead and arsenic. The effects of these " bioactive " substances is, sadly, not limited to the tooth where they are implanted.

Glass ionomers wear much more rapidly than human enamel and they are also, by their very nature, leachable materials. The abraded glass ionomer material will be quickly dissolved by gastric acid so that it's various components will be released in the digestive system.