How do I detect when an item is on top of a specific block?

The command:

Run this command every tick (or every second to save on lag)

execute as @e[type=item,nbt={Item:{id:"minecraft:diamond_block",Count:1b}}] at @s if block ~ ~-1 ~ minecraft:blast_furnace run function namespace:summon_crafter

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How it works:

execute as @e[type=item,nbt={Item:{id:"minecraft:diamond_block",Count:1b}}] Runs the rest of the command as all item entities that are a single diamond block.

at @s Runs the command at the location of any detected diamond blocks

if block ~ ~-1 ~ minecraft:blast_furnace Check if the block underneath it is a blast furnace

run function namespace:summon_crafter Summon your crafter