Exode 30:23

And thou also take to thee the first aromatics, choice myrrh, in weight five hundred minas, and sweet cinnamon of half the weight, two hundred and fifty minas,

MOR DROR’ (FLOWING MYRRH) FIVE HUNDRED SHEKELS. The commentators — including Harav Rabbi Moshe [ben Maimon], 1:3: “Mor is the blood gathered up [in the abdomen] of a certain animal in the land of India known to all, which is used in perfumery.” — have agreed that mor is that perfume which is called musk [an animal perfume]. and mor. But Rabbi Abraham ibn Ezra objected to this interpretation, since [musk] is not a spice [as are the sweet cinnamon and the sweet calamus mentioned here in the same verse], even though it has a pleasing odor. Perhaps this is why Scripture separated it from the spices.” And then [Ibn Ezra] asked: “But is it not written, I have gathered ‘mori’ (my myrrh), which shows that mor is something gathered [like spices], while those who bring musk say that it is a substance gathered in a glandular sac under the skin of the neck of the deer? is not musk as Saadia Gaon said, for the verse speaks of mor being gathered while musk is not “gathered.” Moreover, the verse states, and my hands dropped with myrrh, [and musk does not drop]. But perhaps it does do so, due to its moistness.” [Thus far are Ibn Ezra’s words].
It is possible that we say that Scripture states I have gathered ‘mori’ because mor is the blood gathered up in the abdomen of an animal of the hind species known in the land of India; when it walks between the shrubs on very hot days it scratches against the sac and the blood comes out in thickened mass, which is then gathered from the reed-grass. It states and my hands dropped ‘mor,’ because Scripture imagines its odor to be such that one’s hands drop globules of water because of it.
Others have argued: how could there be included in the incense (myrrh, cassia, spikenard etc.) (Kerithoth 6 a). and the sacred oil the blood of an unclean animal? This too is no question, for that moisture gathered up in the animal because of its abundant blood, which drops from [the animal] whilst still alive, is not susceptible to uncleanness, nor is it repulsive.
The word dror they. have explained to be of the expression, and ye shall proclaim ‘dror’ (liberty), here meaning that it should be free from any imitation or adulteration. Perhaps we might say that Scripture requires it to be gathered when free, meaning that it should be taken from that deer whilst it is free, wandering between the beds of spices and enjoying …

בשמים ראש, valuable fragrances, spices. We know this also from Song of Songs 4,14 עם כל ראשי בשמים. “with all the choice perfumes.” I believe that the word בשמים by itself refers to perfumes and fragrances derived from trees as we also know from Song of Songs 4,16 הפיחי גני ידלו בשמיו, “blow upon my garden that its perfume may spread.” בשמים ראש on the other hand, refers to the sap, or resin from such trees, or to other fragrant plants derived from the soil directly.

מר דרוד, myrrh is globally considered as a valuable source of a fragrance, as is מר עובר, (Song of Songs 5,5) a spice well known and treasured by merchants throughout the world.

ומחציתו, this has been explained in Rashi on Keritut 5

בשמים ראש, “chief spices;” seeing that previously the Torah had written: בשמים לשמן המשחה, “spices for the oil of anointing,” (Exodus 25,6) without specifying those spices, now both the spices and their respective amounts are being spelled out. The word ראש occurs here as number, i.e. “first in rank,” just as it does in verse 12, כי תשא את ראש בני ישראל, “when you count the number, i.e. sum total, of the Children of Israel,” or in ראש חודש, “first of the month.” Compare also Amos 6,6: וראשית שמנים ימשחו, “and anoint themselves with the choicest oils.” Compare also Song of Songs 4,14: עם כל ראשי בשמים, “with all the choicest spices.”