My best answer would be: watch it by yourself!
Seriously, look at existing gema maps: their difficulties, their texture, their length... and you will now what you should do, for example:
-same texture arcibrick everywhere looks ugly
-to highlighted or to dark
-to easy (20 seconds for a gema, this is not a gema...)
-to hard: don't make another impossible_little_gemas, useless...
You can do it if you are as motivated as you seem to be