I'm pretty certain that you can not. I tried to figure out all the tile stuff today, and this is what I got.
[TILE:] is used for workshop definitions, and INORGANIC objects (Stone, soil, rough clusters, plaster etc)
[ITEM_SYMBOL:] is used for INORGANIC objects, to define the shape of BOULDERs that are created from their material.
[CREATURE_TILE:] is used for CREATUREs
[ALTTILE:] is used for CREATUREs with multiple alternating tiles.
[GLOWTILE:] is used for CREATUREs glowing in the dark
[SHRUB_TILE:] is used for PLANTs
[DEAD_SHRUB_TILE:] is used for PLANTs
[PICKED_TILE:] is used for PLANTs
[TREE_TILE:] and [DEAD_TREE_TILE:] are used for PLANTs who have the TREE attribute defined.
[SAPLING_TILE:] and [DEAD_SAPLING_TILE:] presumably are as well, but aren't used in the default raws.
Everything I can see tells me that there's no way to change it at all, especially not per type of gem. Even if I make a custom gem like:
[OBJECT:ITEM]
[ITEM_ROUGH:ITEM_ROUGH_ROCK]
[NAME:rock:rocks]
[TILE:'%']
It sill has the ☼. The same if I use any other tile-related token from the string dump. I'd guess that ROUGH types are just going to be 15 until something changes to make it an exception.
As far as I can tell, the only Item Tokens you can make tile changes to are:
BUILDING (New custom buildings only)
INORGANIC
BOULDER (via INORGANIC)
CREATURE
PLANT
In fact, if you were to make a reaction like:
[REACTION:MAKE_GEM_BOULDER]
[NAME:make gem boulder]
[BUILDING:SOAP_MAKER:CUSTOM_B]
[REAGENT:A:1:ROUGH:NONE:NONE:NONE]
[PRODUCT:100:1:BOULDER:NONE:GET_MATERIAL_FROM_REAGENT:A:NONE]
[SKILL:MASON]
It would give you a boulder made out of the gem which would use the tile that you specified in the base material. But that is definitely tied to boulders only.