It might be hard to change if the text, but it is incorrect grammar to say:
You have received 'A' obsidian
In English if the following word starts with a vowel but does not have a 'U' sound then the indefinite article should be 'an' rather than 'a'.
Hence,
You have received 'an' Obsidian <-- correct
You have received 'a' Turquoise <-- correct
And yes, I am bored.