All depends on luck even If you Take the right biomes.
Like Said above the percentage IS not Always saying the truth!!!
Rolled my self an Emerald Planet which Had 50 percent Emerald and IT was Not mineable !!!
Also did several sapphire Planet Always the Same biomes and percentage was pending between 1 and 2,4 percent… Maze should be the highest biome ,you ll have an amazing silver planet then too and also a great hunting planet, and can gather easy bitter beans.
If you want to have Topas Olympus IS the Key Here Same AS diamonds but Not really huntable.
And Trust me i ve mined so many Planet and compared them how much you get Always in 1 hours
First you Need to know what should your Planet be for ?
Just one Special Things of Mining or you want to combine IT with hunting ? Or gathering ? Or Farming Other Things ?
Dependings what you Re looking for there are Sometimes several Options.
And i would recommend you Always T6 Not T5
If you want to mine you get more Drops and Same for hunting …
And weapons and Tools are Not much more expensive or to Forge…
A Ruby Planet AS example IS only intresting of you want to mine Cooper silver and gold at the Same time AS rubys.
And you use the Planet AS hunting planet too.
You can create this Planet also then for having big spots for lamellas , leaves and so on …
If you Re intrested in Materials Only Take Diamond and you get Tons of coal too so never Spark problems , but its Not for hunting … Or good gathering.