A question was asked in chat from someone who wanted to place a black onyx stone on his or her alter.
So first, lets discuss what black onyx is made because some look at the misconception that onyx is made from cooled lava (a mod said this)
Onyx is a microcrystalline quartz called chalcedony, named for the ancient port Calcedon on the Sea of Narmara where it was first mined. Today onyx is mined in California, Brazil, Uruguay and India. Thousands of years ago, onyx was used to create handles or bases for items made of gold. Onyx also was inlayed with various kinds of stones. The Romans used onyx to create seals made of intricately carved layers. Cameo brooches were also often carved out of onyx. In 250 B.C., an onyx was chosen as one of the 12 stones in the breastplate of Aaron, a vestment made of gems.
Read more: Black Onyx History | eHow.com http://www.ehow.com/about_5413026_black-onyx-history.html#ixzz2DRCiff5R (for referance is where I got the black onyx facts)
Secondly, look at your tradition. If there is no outlined way that an alter should be set up , then it really doesn't matter where it goes.
Finally, the alter is only a tool. The person who has set it up and uses it is the power that is needed. The construction of an alter should be personal to the individual.