From Hindupedia, the Hindu Encyclopedia
The sculptures in the temples should be large enough to be seen by a large number of devotees. The permanent idols are called "Acala" or "dhruvabera" - are made of stones like granite, marble, or soapstone. The other idols or Utsava murthis are made of metal, precious stones, or wood, or clay. The most common metal images are silver, gold, or panchaloka ( five metals). Wooden idols are seen only in two temples: Lord Jagannatha temple at Puri, Orissa and in Trivikrama temple at Tirukkoilur in Tamil Nadu.