I really like the story but I'm still so curious what the title means.....

I might be wrong but this is my interpretation(it's a bit long haha sorry about that)--

Just breaking down the words, 'Mihana' --means a pearl or something beautiful
'da'-- is similar to 'desu', it's like a word that gives the previous word a confirmation of sorts. Idk how exactly to describe it.
'pectolite'-- it's a mineral but for the ones who believe in crystals powers, its used for healing/curing, but idk how it's thought about in this title.

While translation the other language to English, the word placement also can be different. So that could also make a difference in the interpretation. Anyways, i just think it's referring to 'a pearl that's healing', here the pearl being the mc cause he's already good enough and just needed some encouragement to know it.