General

This is probably the easiest way and time saving.. If it's in Reboot what I did
http://global.hidden-street.net/items/quest/star-rock

Went to Arwen (she's in Ellinia) and gave her those materials, after mining them.
I should've kept most of those crystals from ToT, thought it was no use.. And in the end I had to go mine them o.o
Got one without having to go through the effort of hunting those bosses. Ain't nobody has time fo that.

l did it by killing Tae Roon a million times over and over.
lt's not that Snow Witch is being hunted down really fast, it's that it doesn't spawn at all.
Reboot is actually quite glitchy in general in many places. (such as familiars not dropping until after a few weeks after launch)

You can also get the higher up gems from ToT prequests if you wanna knock out 2 things at once.

The npc in Ellinia (Arwen) can easily make a star rock for you if you get these materials (from mining) : Garnet, Amethyst, Aquamarine, Emerald, Opal, Sapphire, Topaz, Diamond, Black Crystal and 15,000 mesos.
Takes less time than constantly ccing and/or trying to farm from mobs.

Its required to finish Alcasters Quest so you can buy all cures for 400 each which is really good on Reboot, and on other servers so you dont have to buy from FM. quest doesn't take long if you ignore the whole star rock being impossible on crowded new server part

do these rocks do something special? or is it a reboot meso thing?

Idk how common it is but I got one casually killing the seashell boss on the 2nd map to the right of Aquarium.