The spell Drawmij's instant summons states (emphasis mine):
Components: V, S, M (a sapphire worth 1,000 gp)
Duration: Until dispelled
You touch an object weighing 10 pounds or less whose longest dimension is 6 feet or less. The spell leaves an invisible mark on its surface and invisibly inscribes the name of the item on the sapphire you use as the material component. Each time you cast this spell, you must use a different sapphire.
At any time thereafter, you can use your action to speak the item’s name and crush the sapphire. [...]
The spell does not explicitly state the sapphire is consumed, which makes me wonder whether the sapphire can be mended with the mending spell1 and then sold. Rules designer Jeremy Crawford replied to a tweet asking whether the sapphire is consumed, stating:
Yep. You crush it: "Each time you cast this spell, you must use a different sapphire" (PH 235).
However, Jeremy Crawford's tweets are not official rulings, and the quote he refers to only indicates that mended sapphire cannot be used as a component to cast Drawmij's instant summons.
Would it be possible to mend the sapphire and sell it to a shopkeeper? If so, would the sapphire still be worth 1000 gp, or would the price be less because it lost the ability to be used as a component in Drawmij's instant summons?
1 I had missed that this would require a casting of the mending spell for every single broken link within the gem. Unfortunately this means mending the gem takes a little longer -- a sapphire broken into 1000 pieces will need to be mended 999 times, which will take about two 8-hour-work days. I'd say 500 gp per day remains worth the cost.