The diopsid native tongue employs their luminous glands, allowing them to flash messages across the largest Underdark caverns without a sound. Diopsids can also create flashes of "light" visible only to creatures with darkvision, allowing them to "whisper" without tipping off their presence to creatures that rely on normal vision.
An observer can detect a diopsid communicating in this manner with a Spot check rather than a Listen check. While creatures that lack bioluminescence cannot "speak" Diopsid, they can understand and translate it.