Actually, in the UK "gneiss" is pronounced exactly "nice", so it fits even better there.
If Wikipedia's phonitic guide is any authority, it's also said that way in the US (hover-text: 'n' as in 'nigh', long 'i' in 'bide', 's' as in 'sigh')... it doesn't make any distinctions between regions. -- IronyChef (talk) 04:51, 18 August 2012 (UTC)