I recently moved to okc from Texas. I know there's some interoperability, and that Toll Tags are accepted. However, I have a TxTag, similar to a toll tag, but put out by TxDot.
Topaz is commonly used for Friendship Rings and it is nicknamed 'the gem of friendship'. Topaz is considered in some cultures to be the ultimate representation of friendship and fidelity.
These symbols of friendship will be a a physical reminder for your friend of the loyalty, caring, trust, and comfort you share.