We have developed a tissue-mimicking phantom for ultrasonic elastography. The phantom has tunable elastic and echographic properties. Polyacrylamide gel, a typical chemical gel in that it shows greater stability than physical gels such as gelatin or agar, provided the basis of the phantom. Fine metal-oxide particles were added to modify the echogenicity of the gel. Independent adjustment of the concentrations of acrylamide and metal-oxide particles enabled fine-tuning of the elastic and echographic properties of the phantom. The tuning was fine enough that we were able to develop a "stealth phantom", which possesses a small hard region that is very difficult to detect in echography but clearly distinguishable in elastography.