Q:

Meaning of 光る事は光るが切れそうもない and 何だ指ぐらいこの通りだと

夏目漱石の坊っちゃん:

親類のものから西洋製のナイフを貰【もら】って奇麗【きれい】な刃【は】を日に翳【かざ】して、友達【ともだち】に見せていたら、一人が光る事は光るが切れそうもないと云った。切れぬ事があるか、何でも切ってみせると受け合った。そんなら君の指を切ってみろと注文したから、何だ指ぐらいこの通りだと右の手の親指の甲【こう】をはすに切り込【こ】んだ。幸【さいわい】ナイフが小さいのと、親指の骨が堅【かた】かったので、今だに親指は手に付いている。しかし創痕【きずあと】は死ぬまで消えぬ。

In particular, I am having trouble comprehending the text in bold. I have  included the context of the sentence in hopes of making it more clear.

ひかる こと は ひかる が きれそう も ない。

From what I understand, it is something along the lines of "the shining thing (the knife) shines, but it doesn't seem to be able to cut". However, what is the purpose of the も particle in this case?

なんだゆびぐらいこのとおりだと

As for this clause, I can't seem to make much sense out of it.

A:

「[光]{ひか}る[事]{こと}は光るが[切]{き}れそうもない」

is in the structure:

「Verb or Adjective + ことは + Same Verb or Adjective + が/けど + Phrase」
= "(Something) is indeed ~~~~, but ~~~~~~"

「切れそうもない」= "does not (even) look like it cuts well".  「も」 is there for emphasis, which is why I used "even".

"It is indeed shiny, but it does not (even) look like it cuts well."

Onto the next phrase,

「[何]{なん}だ[指]{ゆび}ぐらいこの[通]{とお}りだと」

「と」 at the end is quotative.  So, for the sake of understanding, do pretend you are instead seeing:

『何だ指ぐらいこの通りだ！』と（言って、言いながら, etc.）

何だ = "what the heck", 指ぐらい = "it's only a finger", このとおりだ = "just like this" ⇒ "I'll show ya."　
This is what the guy said before he actually cut his finger.  You should be able to figure out how to put it all together as an indirect quote within the sentence.

