Gems spawn in clusters of types so if you find a ruby, youll probably find more rubies in that area.
You have a chance with every wall you mine to uncover a troll or cave slimes so be ready to sprint away always.
You often can get a much higher quality stone axe than pickaxe earlier on so its usually better to mine for quality with stone axe.
In general mining is pretty straightforward, just spam pillars and mine in 1 direction endlessly with random side bits like --|--|--|--|--|--|--|--|--|-- The hardest part is grinding strength tbh. You dont need to do side shafts every 2 tiles like minecraft because ores usually spawn in clumps so I do 1 side shaft per pillar normally.
Different rock types have different strengths. Youll get used to each type eventually but some are just a hard miss until way later on, you have to make a lot of detours till you have chad stats.
You usually dont want to make mineholes directly ontop of prospected ore because youll clear half the ore node, you should make it to the side and use knowledge of hardness to try and land on something you can actually mine, its also good if you get "there is nothing below" because its a minehole straight into a cavern or tunnels.
Also have a bat cape because fighting every single bat is a pain.