Tested in SSP. Use the Ore Dowsing Rod to highlight ores, then save and quit, then rejoin the game. The highlighted ores are now permanently highlighted, even if the ore is mined.
I haven't figured out how to get the Dowsing Rod to force drop the highlight.
I wanted to use it to find certus quartz because I haven't found a single block in over two months of play. Any tips on how to find it because progression in the magic mods is gated behind getting a fair amount of certus quartz.