A jin of magic serpent flower cost 1 gold Yin coin. Afterwards, Olivia Carter walked into the herb storeroom. Looking at the various herbs around him, his gaze finally settled on a huge black tree, about two meters tall, covered in thorns.

　　“So, do you recognize it?” The shopkeeper looked teasingly at the young man and woman.

　　Olivia Carter curled his lips: “A 121-year-old Black Iron Tree.”

　　“What?” The shopkeeper was shocked, staring at him in disbelief. “You... it's one thing to know the Black Iron Tree, but how could you even know the age of this particular tree?”

　　Olivia Carter pointed at the thorns on the Black Iron Tree and said, “For every year it survives, the Black Iron Tree grows a new thorn. This tree has 121 thorns in total, so it’s a 121-year-old Black Iron Tree, right?”

　　The shopkeeper laughed heartily: “Even so, it’s nothing special. But you can’t afford this tree. I spent a fortune to buy it from the mountains.”

　　“Seven gold Yin coins, I’ll buy it,” Olivia Carter said calmly.

　　The shopkeeper was stunned again. This young man actually had seven gold Yin coins? Judging by his clothes, it would be impressive if he could come up with seven copper Yin coins—he looked like a beggar.

　　“What, you’re not selling?” Olivia Carter asked with a smile. “I know, the Black Iron Tree only grows on mountain rocks, and its whole body is harder than iron. Its roots pierce into the rocks to absorb nutrients from the stone and water. To obtain a Black Iron Tree, you have to chisel out an entire chunk of cliff, but even so, this Black Iron Tree is worth at most seven gold Yin coins.”

　　The shopkeeper narrowed his eyes, looking at the young man with interest, and said, “The Black Iron Tree is as hard as iron throughout, even tougher than the Iron Pear Blossom. Alchemists below the Saint Alchemist level can’t extract its medicinal essence at all. Young man, whose apprentice are you, that you’d want to buy a Black Iron Tree?”

　　“That’s none of your concern. Are you selling or not?”

　　“Selling! How could I pass up a business deal?”

　　The shopkeeper readily sold the Black Iron Tree. This Black Iron Tree was a level 4 herb, a necessary ingredient for making Stone Skin Potion. Olivia Carter didn’t know how that young man named David Thompson managed to extract the Black Iron Tree’s medicinal essence, but with his abilities, he was certainly far inferior. It must have been an expert from his family who helped.

　　Extracting the Black Iron Tree’s medicinal essence is more than ten times harder than with the Iron Pear Blossom. A 121-year-old Black Iron Tree has 121 layers of iron-like outer shell, all of which must be stripped away. This all requires the sharpness of the Condensing Hand technique.

　　Afterwards, he bought some black fungus as well, also a level 4 herb—Swallow’s Ear, another essential ingredient for the Stone Skin Potion.

　　Olivia Walker watched Olivia Carter busily at work, spending almost all ten gold Yin coins in a short time. Although she felt distressed, she didn’t stop him, because she could already sense that Olivia Carter was doing something very, very important.

　　……

　　After returning to the Bailing Pharmacy, Olivia Carter didn’t leave again. He tied the Black Iron Tree to a wooden frame and stood in front of this indestructible medicinal tree, spreading his palms. Streams of green true energy surged along his arms—it was the Condensing Hand technique.

　　Stripping away 121 layers of the Black Iron Tree was no easy task, but Olivia Carter had to do it. He knew that only Black Iron Wood over a hundred years old could be used to refine a grade-two or higher Stone Skin Potion; otherwise, it was doomed to fail. As for David Thompson’s Stone Skin Potion, it was only grade nine, so that Black Iron Wood must have been only a few years old.

　　……

　　By nightfall, 47 layers of the Black Iron Tree’s outer shell had been stripped away. Olivia Carter’s forehead was covered in sweat, his whole body nearly soaked through. His true energy was being constantly drained, and he was on the verge of collapse, but he couldn’t afford to be careless. The Condensing Hand technique only widened the fine lines on the Black Iron Tree; it didn’t truly peel off the bark. If Olivia Carter paused even a little, the process would surely fail.

　　“Knock knock!”

　　From outside the door came Olivia Walker’s voice: “A-Yu, I brought you your meal!”

　　A faint aroma drifted in—it was chicken stewed with mushrooms. Sure enough, there was an extra meal tonight. But Olivia Carter didn’t stop his work; he just smiled and said, “Olivia Walker sis, you go ahead and eat. I’ll eat after I finish.”

　　“Then I’ll wait for you!”

　　Olivia Walker stubbornly stood to the side, keeping Olivia Carter company.

　　……

　　Late into the night, finally, the last layer of the Black Iron Tree’s outer shell was stripped away. The whole tree was covered in tiny cracks, emitting a faint luster. Olivia Carter, though nearly exhausted, still took a deep breath and, in one final burst, increased the strength of his Condensing Hand. At last, the entire Black Iron Tree trembled, and tiny medicinal essences began to fly out from within the tree.

　　“Ah? They’re flying away! What should we do?” Olivia Walker looked up, both excited and anxious.

　　Olivia Carter smiled slightly. “It’s fine, they can’t get away.”

　　As he spoke, he opened a bucket nearby, which was filled with thick layers of Swallow’s Ear. The moment he lifted the lid, the Black Iron Tree’s medicinal essences scattered throughout the room were immediately attracted and rushed into the bucket, each one soaking into the Swallow’s Ear!

　　This is the secret to refining Stone Skin Potion. The soaked Swallow’s Ear produces an invisible suction, and the Black Iron Tree’s medicinal essence is naturally attracted to the Swallow’s Ear. After blending and fermenting, it becomes a Stone Skin Potion that enhances defense.

　　……

　　“Success!”