Holy, holy, holy, the Lord God the Almighty, who was and is and is to come.
I really like this because it reminds me of the account in Isaiah 6 when Isaiah has the vision of the temple, where the angels again proclaim "holy, holy, holy". This is an important parallel because both are considered books of prophecy by the authors and both are accurate accounts that line up to each other.
And the one seated there looks like jasper and carnelian, and around the throne is a rainbow that looks like an emerald.
This description of Jesus and the throne reminds me of the description of the Son of Man in Daniel.
Come up here, and I will show you what must take place after this.
This is really interesting to me because it shows that everything that happens in life must be the result of something else, so in this instance (presumably) Jesus is saying that things must happen for other things to take place.
The words of the Amen, the faithful and true witness, the origin of God’s creation
Here we see the amazing parallel that He is the first and the last (the beginning and the end). This is a theme that is brought up multiple times throughout the entirety of Revelation.
who has the key of David, who opens and no one will shut, who shuts and no one opens
This is interesting to me because it touches on multiple subjects, but the main theme seems to be that Jesus can do what no one else can. This seems to make mention of the Isaiah parallel as the "root of David".
These are the words of him who has the seven spirits of God and the seven stars
This is interesting because it goes back to the mention of the seven stars from chapter 1, the ones that the Son of Man is holding.