I can't seem to find any gems!
Unlocked at Level 8, you can change your bonus meter to award gems every time you fill it up.
To change the power up the meter gives, select "Power up" on the character selection menu, and the select "Gems".
Power ups (like gems) obtained this way still count towards daily missions and multiplier objectives.
Essentially, fill the bonus meter five times with the gem power up equipped.
I can't seem to find any artefacts!
It seems like RNGesus wasn't on your side...
Artefacts are a rare power up. You can increase your chances by levelling up your "Pickup Spawn Chance" passive. When upgraded to Level 6, it increases the spawn chance by 30%.
Alternatively, play on the limited time maps (unlocked with 500 gems outside event), as they have greater chances of spawning Relic Chests. While these chests mostly contain the level's relic set, it also has a chance to spawn regular artefacts.