The Church does not approve or condone the use of crystals in any spiritual context. All practices related to divination of any sort are forbidden, because they either rely on superstition or potential demonic influence.
There are some minerals and rocks that are said to have magnetic properties that are beneficial from a physiological standpoint, but that is viewed strictly in a medical or therapeutic context without reference to anything spiritual.
Crystals, however, have been co-opted by the New Age movement to channel energy from the universe, which is a euphemistic way of describing a pantheistic worldwide view of life, which is contrary to the revealed truth given by Christ.
God is transcendent of the universe, not simply an intrinsic consciousness or energy thereof.
So, decorations, yes. Spiritual device, no.