It does not matter if you use seven identical items so long as one item is from a different group. At far as I am aware, there are four groups:
So, there is no reason why you should not use seven common relics and one gem or metal or meteor.
By the same reasoning, you could use seven oval cut rubies and one meteor. Etc.
The only thing that you must ensure is that you have a combination valued between 3,500 and 4,999 and that you have at least two groups represented.
I find this is a useful way of using up common relics and an excess of emerald cut gems. Use a number of items of similar and high value (example: 3 valuable gems adding up to 3,500) then you make up the remaining five slots with five bits of "junk". This makes counting easy!
(Metals may be best transfigured into higher value metals first. Example, place two pieces of medium or high value metal of the same type in the first slot, then fill up the remaining seven slots with seven pieces of low value metal (one per slot). This should produce one high value piece, one medium and possibly one piece of iron. The result is not entirely random, but, to try to list the most likely outcome is a big ask. However, you almost invariably end up with more value than you put in.)